IPv6 addresses aren't confusable with a DNS name in these syntaxes AFAIU so it's not that they didn't consider IPv6 but that it's not relevant to the issue.

Yes, "Don't stand out" technologies like ECH aren't useful if you inherently stand out anyway. They're intended to make broad surveillance and similar undirected attacks less effective, they aren't magic invisibility cloaks and won't protect you if you're a singular target.