I don't mean to defend Facebook but Instant Articles is based on RSS... hardly a walled gardern. On the other end, AMP is (yet another) re-invention of the wheel from Google.
I guess the upside is that I also wouldn't see content in the <amp-ad> or <amp-pixel> elements.
I wouldn't disagree with AMP being "just web technologies," but it's sure as hell not "just HTML."