i don't think htmx is very opinionated: it generalizes hypermedia controls in HTML and that's about it. No strong opinions on how you use it beyond that, can work reasonably well w/ any back-end that produces HTML. The goal is to expand the set of UI use cases that can be cleanly expressed in HTML w/o resorting to (user) scripting.
Yeah, htmx is 3800 LoC, and you could do an 80/20 version of it for a lot less, but there are a lot of details to get right: history, collecting inputs, etc. plus, since it's a general purpose library, it has an extensive event system, extension mechanism, etc. There isn't a ton of dead weight to drop, although 2.0 should be smaller as we drop some bad ideas in 1.x.
I don't care too much about the holy wars around REST, I found them off-putting back in the day before I really understood the concept, but I am passionate about the uniform interface and communicating to developers why that was interesting and different. I do go a bit hot at times, especially at twitter, but, on the other hand, if I didn't, would you have ever heard of htmx? I try to balance that all out w/ reasonable essays and the book, which, I think, is worthwhile for most web developers to read (it's free online.)