While true, they also require user agents to support other encodings specified that way: https://html.spec.whatwg.org/multipage/parsing.html#characte...
Another funny thing here is that they say “but not limited to” (the listed encodings), but then say “must not support other encodings” (than the listed ones).
It says
> the encodings defined in Encoding, including, but not limited to
where "Encoding" refers to https://encoding.spec.whatwg.org (probably that should be a link.) So it just means "the other spec defines at least these, but maybe others too." (e.g. EUC-JP is included in Encoding but not listed in HTML.)
Ah, I understood it to refer to encoding from the preceding section.