"Hey useragent, if you understand this markup, please send mobile users to this other URL that I have provided."
Twitter then respects the website owner's markup request.
If the markup was "here's a mobile optimized page", we wouldn't get this kind of strong reaction in HN. If the markup was "here's a the language X variant of the document", we wouldn't get this kind of reaction in HN.
Edit:
It's a complicated situation. Google offers a great carrot/stick to get people to adopt AMP. Publishers adopt it. Other platforms start using it. A lot of users like this format. Some users hate it and the idea of it.
As I've mentioned before, I do think that Twitter was trying to do the right thing for their users given the situation we are in.
It would be nice to have "request Desktop version" option for AMP, but not sure if it's too much to ask.
You would want to look for <html amp> to detect this case, and then <link rel=canonical> to determine the possible non-AMP URL. If that URL differs from the current URL, redirect to it.
If you personally want a per-user option here to override the publisher, this seems like the way to go, but there is some similarity to ad-blocking here.
Since when is Twitter the user agent?