The point here is not concerned with the specifics (which is what Tilt seems to be caught up in) but that for anything to 'exist' it must remain so for some period of time, or else it cannot be said to be, for it is already something other. Flux presumes perpetual change and that breaks the principle of self identity, for if flux were true, then nothing would in fact exist, because it would not 'be' at all, it would be otherwise.
The assertion "it is" is in perpetual conflict with it being otherwise. That is why it is dukkha. If we hang onto the idea that identifying things is tantamount to declaring their actuality, we are hanging onto dukkha.
Now you might say that flux is true in the scientific sense that things are in perpetual motion at a minute level that is far beyond our perception. For example that a chair may 'appear' to be the same chair it was a minute ago, but it is changing all the time, at an atomic level (or however you want to slice it). However to us, the chair remains the same until it changes.
We have an idea of the chair not changing. What is going on in our experience is nothing like that idea.
So we have a problem, we can either assert the existence of the chair, or we can deny it.
Actually these are not our only options. We can notice what is actually happening and understand if talking about the chair is useful.
To assert the existence of the chair, to say that the chair exists in my experience is to deny the idea of perpetual change. To deny the existence of the chair is to say that although it 'appears' to be a chair, it is in fact in perpetual flux, along with the rest of our world, and we do not see that because we are ignorant of the Buddha's teaching.
We dont see flux because the object we pay attention to is our idea "this is a chair". I think the Buddhas teaching is far deeper than merely noticing flux.
Now if you re-read the first quote, you will see that this is nothing more than the two contentions that the Mahayanists make. It is the same argument, unfortunately that Orthodox Theravadins make.
Teachings, whether Theravada or Mahayana have there own contexts which make them not the same. I think the middle way between
we can either assert the existence of the chair, or we can deny it.
can be found in either tradition.