There's a spectrum on this; a lot of stuff in the async ecosystem uses mkdocs (FastAPI, etc.) for example. I'd also argue that Markdown is known by waaaaay more SWEs than Sphinx--I wrote a ton of Sphinx years ago and couldn't even tell you where to start today, but I'm fluent in Markdown because I use it everywhere. That says a lot.
Looking through the Textualize docs [0], they seem pretty good to me? It reminds me of most of the web UI toolkits I've used: components on the left, specific functions of the component you're viewing on the right. I think the advantages to Sphinx you're pointing out aren't super relevant to Textualize.
My day job's in Python, and I spend a lot of time reading through various Sphinx documentation. Sure it's possible to make a great Sphinx site, but most people just add bare minimum docstrings and leave you to figure out the rest. Like you say, people generate docs, and I don't think Sphinx has extra magic that "makes you" generate better ones.
On the contrary, I think something that does help you generate better ones is liking your doc tool. If you're not into Sphinx, you're not gonna be enthused about writing excellent docs in it. If you're into mkdocs, you'll be more motivated to do so. The best exercise is the exercise you'll do, etc. etc.