The assertion was that there is too great a diversity of device sizes for a LaTeX-based document standard to work, with the strong implication being that a fixed-size PDF is the only output option for LaTeX.

Both sides of that fail.

You're now shifting the goalposts.

Yes, ePub is based on HTML. It is also contained, standardised, and structured (not dissimilarly to how LaTeX itself is based on Tex but with standards and structure). It's also not the only fluid document standard, though the other I'm significanlty aware of (mobi in particular) are effectively proprietary.

ePub exists and is good enough.

It has some issues of its own, including an overly-strong foundation in HTML which could well lead it to many of the same issues plagueing the Web. In practice, to date, it's largely avoided those pitfalls.

But again, that's really not what the question I was answering was about. Rather it was in targeting multiple sizes of devices. And I think I somewhat addressed that.