There isn't any rationale. The author doesn't even attempt to explain the reason behind most recommendations.

The author doesn't justify it but I suspect the thinking is related to how fixes are prioritized.

If there is a bug/performance issue that only happens in the context of AWS, then Corretto would be the most likely to rapidly patch the issue as it impacts their customer base. Potentially patches for those sorts of issues would be discovered by AWS through their own use and testing.

Likewise for the other vendors and their own platforms.

I had no idea anyone thought Corretto should be isolated to AWS. I've found nothing wrong with it.

I'd also like to know. Even some large Android projects use Corretto.

Nothing - but any of the "vendor JDK" they recommend on that vendor's platform only - see RedHat for another example.

And a similar question for the Microsoft build. This isn't Azure or Windows specific at all.

I had the same thought, what’s the reason this is not recommended outside AWS?

Not sure. The JDK works pretty well, but for whatever reason, I had issues with Corretto crypto provider and running it with Metabase. Switched to Adopt and didn't look back