Ah, I see. I actually originally posted the x.com links but plenty of HN users can't read Twitter threads that way, so we usually try to provide a readable alternative. (Plus this helps to reduce offtopic complaints in threads.) You'll therefore often see both a twitter/x link and the corresponding xcancel link in toptext - like in these recent examples:

https://news.ycombinator.com/item?id=48317774

https://news.ycombinator.com/item?id=48311485

https://news.ycombinator.com/item?id=48212493

https://news.ycombinator.com/item?id=48071262

In the current case, I decided not to do that because it would have made the toptext way too noisy, so I chose the domain that most people would be able to read.

As for HN's policy, you're right about the rule but not its scope. It only applies to submission links, i.e. the URL that a story title is linked to, which also determines the domain displayed to the right of the title.