The fact that people here on HN are still requesting it says it's far from well implemented - I wouldn't have known about it if I hadn't read it somewhere when you added it. The more conspiratorial among us probably assume that implementing it poorly is intentional.
Is it because "Sharing" in this case us meant to imply pushing the content to another medium (such as an external browser) vs. just the implied meaning of "sharing with another person?"
I'd love any context you can share from internal discussions around how prominent to make that functionality. I think it is hard to deny that Google benefits from it being harder to find, which is where the criticism from the parent is likely coming from.
I literally loaded Chrome for Android after reading your comment, searched for a Reddit thread, and no button, nor any other way to bypass AMP without having clicked the AMP link already.
Screenshot: http://i.imgur.com/Xnrlzcs.png
Before, there was no way to get the original URL. Now they display a "link" icon on the top so you can get the original URL.
From the results themselves, no, there is no way to bypass AMP (like there is with App-indexing)
Also: using browser's native sharing option, shares the Google URL's, not the "real" one