Interesting info, but whether or not it's true, it certainly benefits their strategy. I have no doubt that they want to push the narrative that Google (Search) is the 'platform' and the news from publishers they wrap is a tab or viewport that you can [x] (exit) out of, returning to the default 'blank' Google page.

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].

[1] http://imgur.com/a/vAtqq