Yep, it's opt in for publishers. For users, the only way to opt out currently is to view in desktop mode.
I now use https://github.com/bentasker/RemoveAMP, but that's only possible because I'm lucky enough to have a Jailbroken iPhone. :/
I guess you'd call it an "extension"? It feels weird to use that term, as it implies (to me) that there's some sort of extension framework in place. There isn't, it's just code injection.
Yeah extension does sound weird.