Wow, this is just terrible. Sites using this will now be indexed by google, but not by any other search engine, since it's encrypted with a key only google [and subscribers] can read. I really hope AMP never gets any large scale adoption by consumers.
I remember in the early days of Google, it was a big no-no for sites to serve different content to Google than to actual users. Sites caught doing this would be banned from Google, so SEO advice quickly became widespread: don't do this, it will hurt you. But results like this seem to be commonplace now, with Google doing nothing about it. This article seems to be overt confirmation that Google no longer has an issue…
Google isn't encouraging anyone to deliver different results to Google. It is providing a solution that serves the same content to all users (and search engines). That is exactly the thing you said they were espousing previously.
Previously, the actually worthwhile solution to paywalling was to not load the content at all until after auth. This means that you were showing different content to a google spider vs. a subbed user. Now it's the same.