The problem is not the download size. The problem is the amount of code to maintain.

It's both. All the JS code has to be parsed and interpreted by web browser, which is affecting especially low end smartphones. Better use plain HTML and CSS whenever possible, see e.g. PHOOOS: https://github.com/niutech/phooos (demo: https://kodus.pl)

I don't think the author of the original comment would really care about the download size, given that it was an "internal self serve web app".

htmz looks very cool, though.