That's my main pet peeve with AMP as well. In my mind the [x] in the top bar is almost exclusively associated with 'remove the bar' behavior in Safari on iOS.
I really think AMP can benefit a lot from PR stand point by letting techies opt out from it, but leaving it on for "regular" users. See https://twitter.com/slightlylate/status/820344221450125312
The problem with that is that the UX is inconsistent between AMP results and web results. AMP results pretend they're this fake tab that can be "closed" and you return to Google, while web results take you away to the URL like you'd expect, which means navigation is the responsibility of the browser, and not of Google Search.
Frankly, this all would make more sense in the viewport-wrapping context of Google Chrome or Google News or the Google App rather than a behavior of Google Search, but that's not the limit of their ambition.
Or just, y'know, dress up the results page a bit more, and be honest about it. Brand the AMP subsection 'Read Instantly with AMP' and have it clearly and visibly wrap the results. This gains them a bunch of mindshare.
Or change the [x] to a 'back arrow', like their own splash page shows [1].
Google is search, everything else is secondary. 20 people in a room testing this should have been all the red flags they needed to fix it before launch.