I'm coming from a Deleuzian postion, but I don't think it's really a counter. The moves made by SX suggest to me he's been trying to talk about a certain kind of relation to us all along. I don't think they're after existence itself (difference itself), but a specific kind of relation formed by these differences.
SX is talking about the concepts we have beyond pure difference, certain conceptual relations of form and how they are defined. The point, I think, is these conceptual distinctions mean something. They aren't just an arbitrary whim of an interprating actor. Scepticism wins no battles here.
We can use pure difference to understand this consequence. Difference itself is the present of an object. If something is so, difference has defined it, amounts to the presence of a thing (and its absence; it will be unless another difference comes along).
At this point must bring formal relations back in. The object I'm considering may be pure difference and present only in difference itself, but that's
not all it is.
These objects, these differences are actors and performers upon our plane of forms and phenomena. They aren't just differences, but differences which do something in relation to each other, in relation to us.
The screen I'm looking at, for example, is a pure difference. It's not existent by its form. At any moment it might disappear or even turn into a flower. I cannot use the forms I expect of it to judge whether it exists.
This, however, doesn't mean my screen is without form and its impacts upon me. My screen may possibly do anything, but that does not mean it does nothing. On the contrary, the difference of my screen does a lot of very specific things to me. It's my visual interface for communicating messages on this forum, for example. If it turns into a flower, it will do other very specific things to me.
In any case, the screen, the pure difference, the screen is acting upon me in certain ways. The difference (the existence of the screen) has consequences for my experience and my relations. The difference and its effects, are neither a whim of language (Derrida) nor ultimately mysterious and inaccessible (Heidegger).
In short, I agree with what SX is saying. I just think they've misdiagnosed the object of their critique. SX isn't attempting to talk about existence itself, to get beyond any talk of relations. They are trying to talk about the impacts
of a difference itself (a thing that exists) upon the world of relations, the forms and relations a given difference performs in its presence.