It’s worth reminding that browsers have stopped sharing caches across domains years ago, because cache state could be used for cross-site tracking.
So a shared fonts domain doesn’t help. With H/2 and H/3 it’s best to send everything from one domain.
why is it worth mentioning when the article doesn't say anything to the contrary?
The article doesn't mention cache specifically, and it the past having a shared cache and extra H/1 connections has been the justification for using 3rd party domains.