That's quite a big assumption from one usecase without much evidence backing it up. I'm not saying the author is lying but I'd need more data before saying this is a repeatable scenario...

Even the author couldn't narrow down what exactly was the cause other than maybe:

a) the domain name being different

b) the addition of chrome w/ a link to Google support + a notification "You're now logged in as [user]@gmail.com"

c) minor CSS changes as a result of minimizing static assets

None of these screams an obvious problem, especially given the other benefits this process has given in return (which of course could be accomplished without AMP).

A conversion rate drop doesn't necessarily mean it's unpopular with users, just that people aren't converting.