This has been discussed before. XT is an altcoin, and services that move from Bitcoin to XT are altcoin services. On bitcointalk.org, altcoins and altcoin services belong in the Alt Cryptocurrencies section. On /r/Bitcoin, altcoins and altcoin services aren't allowed.
Promoting XT and moving to XT is not the same thing.
That would be like banning a service, because they consider moving to another altcoin, even though they are still using Bitcoin.
I just want to say that I agree that it seems like he's going even farther here in that you might not even be able to talk about a company who promotes XT. As long as XT and core are running on the same network, then it seems totally outlandish to say "you can't even talk about company X because company X has known opinions which are contrary to mine". I dunno, it seems like another step down a road towards nonsense. Next thing, anyone who's ever shown sympathy or spoken kindly to someone who was a known XT promoter is not allowed.
EDIT: for the record, I'm not really an XT supporter, I run core.