I read the post top to bottom and it never mentions anything but changing the url. It doesn't say that top AMP banner thing is going away.
That's how they're able to display the original page's URL in the browser without that practice being extremely misleading.
If the bar is going away Google can help themselves by saying that specifically. I'm not the only one that hates that thing.
In particular, one obvious way to implement this would be to have everyone sign a static webpackage that includes a JS loader that pulls resources from www.google.com/amp/ via XHR/CORS (using ServiceWorkers I guess?), which would only need to be signed once, and send that over to Google. Then the infrastructure continues to work the way AMP does today - you just upload actual AMP-compatible web pages to your actual website, and Google downloads and caches them. Probably this webpackage would just include a single script tag that pulls the loader itself from Google, so that Google can apply updates to the loader without having to bother you again.
The webpackages spec talks more directly about signing the actual data, which would be way better for the web, but it's not obviously better for Google and for publishers already invested in AMP, so we should be cautious that maybe Google does not mean this and means something that's easier for them and retains the lock-in.
And in terms of functionality, does the AMP web version have all the functionality of the normal site? Often this is something I see missing on AMP sites. A top-bar gives you the ability to get back to the full site.
I think it's the other way around - webpackages include Expires: headers for each resource that are signed by the origin, so the browser decides whether to trust it or not, or maybe to render it anyway but show a staleness indicator. But if you're using a CDN, the CDN implements caching on its own (hopefully following the origin server's instructions or the customer's instructions in a control panel, but no guarantee) and all headers are entirely controlled by the CDN.
> And in terms of functionality, does the AMP web version have all the functionality of the normal site?
On the technical side, this is now the origin's decision, not Google's - they can include whatever functionality they want or whatever links they want in the webpackage. It's not really different from having a mobile site that has a "View full site" link, or worse, doesn't have one and also misdetects your browser.
I think I agree that in practice it matters a lot what people choose to do, and whether Google adds restrictions for what sorts of webpackages count as "AMP" for the purpsoe of special treatment on the search results page.
(Also, if we think this is worth indicating, it should be done by the browser itself, not by the web page voluntarily including some CSS.)