You are on a search page, you go on layer deep by clicking. The close button goes back. I don't see what else it should do. Mind you that I do understand that a link to the origin server would be a good improvement (and I've stated that in the post).
I'm not sure what you find disingenuous. AMP are just web pages saying "HEY I A AMP, please use treat me as AMP". You can publish the same bytes minus the "HEY I A AMP" and get the old behavior.