In fairness AMP does not have a requirement to use a CDN. It allows the use of a CDN which was another reason why Google pushed it (to do all kind of same-domain tricks), but to keep on topic, I actually don't think Twitter used that Google cache.
"As a publisher, you don't choose an AMP Cache, it's actually the platform that links to your content that chooses the AMP Cache (if any) to use."
"Caching is a core part of the AMP ecosystem. Publishing a valid AMP document automatically opts it into cache delivery."
https://amp.dev/documentation/guides-and-tutorials/learn/amp...
In this case Twitter is the platform, not the publisher, and would absolutely have been able to not use the Google cache.
> Now, when using one of Twitter's mobile clients, users will be sent to the amphtml URL in their browser, instead of the link that was shared in the Tweet. Users will load this link directly, not via a page cache. [0]
[0] https://developer.twitter.com/en/docs/twitter-for-websites/a...
> it's actually the platform that links to your content that chooses the AMP Cache (if any) to use
Twitter (the platform that links to AMP content) could choose which AMP cache if any to use.
Oh, I guess maybe the confusion here is that your argument is that as a page publisher you cannot opt out of the Google CDN? I read your original statement as "it is not possible to view AMP content without using the Google CDN"