I believe the point of JTB is not to arrive at certainty, but merely to filter out bad or unreasonable beliefs. Even if the justification criterion were modified to only allow true propositions for justification, I don’t think that would entail certainty. So it’s a practical definition of knowledge.

The traditional JTB definition cannot guarantee knowledge, only reasonably true knowledge. Just because you’re justified in believing something doesn’t mean it’s necessarily true.

That’s what certainty is: If there is no way you could be mistaken about your belief, then that belief is certainly true, e.g. ”I think, therefore I am”.

I’m not saying that it is an absolute truth. What I’m saying is that right now there is no possibility that the belief is false. Being a thinking thing is not necessarily an absolute truth because when you die you will cease to think.

Yes. I’d say you don’t have true knowledge unless there’s absolutely no way for that belief to be false.