Question: why is `https://orgmode.org/` in html and not in ... org mode?

It is [0]. All the pages are generated from org sources. If you scroll to the bottom of any page there is a link. Also tec used to have an overlay that would show you the org source directly, but ultimately it was decided that it should not be enabled because it required javascript and could be confusing.

0. https://git.sr.ht/~bzg/orgweb

Looking at the markup of some pages, such as https://orgmode.org/Changes.html , at least parts of the website seem to have been generated from Org mode.