A fallacy is generally an error in logical argument, while equating T.I. with solipsism is merely a gross misunderstanding.

Probably shouldn’t try to identify a fallacy without the argument from which it may arise.

I agree with Mww. However, if you want to put into perspective how T.I. relates to solipsism (S), then you may want to consider that:
S denies the existence of anything outside the mind.
T.I. makes no claim what's outside of the mind.
T.I. includes the belief of a possibility (without providing a probability) that our perception is actually that of the outside world.
T.I. includes the idea that some of our observations may be right on top of reality, while other of our observations are dead off target.

A fallacy is generally an error in logical argument, while equating T.I. with solipsism is merely a gross misunderstanding.

Probably shouldn’t try to identify a fallacy without the argument from which it may arise. — Mww

Thank you both for the replies. Essentially there is not much more to the argument, but the chap has just defined TI in terms of solpsism, and then carried on with a discussion substituting "TI" for "solipsism".

It seems to fit the definist fallacy since this fallacy essentially consists in (1) defining one concept in terms of another concept with which it is not clearly synonymous, (2) as the persuasive definition fallacy, defining a concept in terms of another concept in an infelicitous way that is favorable to one's position.

In my above example, transcendental idealism (concept 1) has been defined in terms of solipsism (concept 2). These two concepts, two my way of thinking, are not in the least synonymous.

Could I be missing something else?
Thanks again.

False equivalence? Dunno....I’m not much of a logician.

I’d like to see those definitions, though, bearing in mind CPR is an epistemological treatise, in which the plurality of human subjects on the one hand, and the necessity of the material world on the other, is given, along with the possibility of other rationalities contained in it. How one gets from that to solipsism is beyond the scope of the work.

False equivalence? — Mww

Thanks again, my friend. I actually also was thinking False Equivalence, but it seems a bit different since with False equivalence I thought there has to be at least one characteristic in common between the two things. But I really don't see the correspondence between "TI" and "solipsism"! And Kant never espouses solipsism in any sense, and TI does not imply solipsism.
Both transcendental idealism and solipsism are defined terms and he comes along and basically tries to equate the former with the latter.
Keep in mind too, this commentator is trying to refute Kant's metaphysics...

the definitions:

"Transcendental Idealism purports that the real self is noumenal and space and time are merely faculties of the brain, and that ultimately only the noumenal "I" exists". Let us call this view "Noumenal Solipsism", that is, the view that only the noumenal "I" exists."

With respect to “the real self” as noumenal, the definitions are opposed to what Kant himself posits. Briefly, without the backdrop which contextualizes it:

“...Now, in this intuition the thinking self must seek the conditions of the employment of its logical functions as categories of substance, cause, and so forth; not merely for the purpose of distinguishing itself as an object in itself by means of the representation "I," but also for the purpose of determining the mode of its existence, that is, of cognizing itself as noumenon. But this is impossible, for the internal empirical intuition is sensuous, and presents us with nothing but phenomenal data, which do not assist the object of pure consciousness in its attempt to cognize itself as a separate existence, but are useful only as contributions to experience...”
(CPR, B430 in Kemp Smith, 1929)

With respect to space and time being “faculties of the brain”.....that just doesn’t work at all from a Kantian position, first because Kant doesn’t discuss the brain other than incidental mention, and second because the brain is substance, which has no faculties qua faculty, but has only material cause and effect as functional predicates. The author has taken unwarranted liberties, for a faculty of the brain is very far from “the subjective constitution of the mind”:

“....What then are time and space? Are they real existences? Or, are they merely relations or determinations of things, such, however, as would equally belong to these things in themselves, though they should never become objects of intuition; or, are they such as belong only to the form of intuition, and consequently to the subjective constitution of the mind, without which these predicates of time and space could not be attached to any object? In order to become informed on these points, we shall first give an exposition of the conception of space. By exposition, I mean the clear, though not detailed, representation of that which belongs to a conception; and an exposition is metaphysical when it contains that which represents the conception as given a priori....”
(ibid B38)

Your author may have some points to make, but it would appear he has nonetheless begun his theory under a misappropriation of terms. And I concur, insofar as T.I. properly understood gives no ground to admit solipsism.

“the real self” as noumenal, the definitions are opposed to what Kant himself posits. — Mww

essentially to the extent that it is not accessible, much less a "noumenal solipsistic self"....

Your author may have some points to make, but it would appear he has nonetheless begun his theory under a misappropriation of terms. — Mww

Well it just appears so sloppy, that I don't know where to begin, and the idea that "TI" is some kind of noumenal solipsism is just way off base...but mind you that by him equating "TI" with solipsism, it puts him in a better position to criticize it....like a straw man (if that's the correct term?)

It goes without saying anything can be criticized if the definitions grounding the criticism are in opposition to the definitions given in that which is being criticized.

Kant apparently didn’t find it worthwhile, in the necessary certainty of “I think”, to attribute that certainty to humanity as a whole. In other words, just because I am apodeitically certain I think, does not grant me the authority to thereby claim that I am the only one who thinks. Even so, it should be readily apparent that any agency operating under a given system, speculative or otherwise, would necessarily operate in accordance with that system. Which immediately falsifies the Enlightenment notion of solipsism.