threads supposedly intends to eventually federate with activitypub, so it's entirely possible the bbc is just betting on the one with the more active users, assuming eventually threads will pull in their feeds anyway.
Edit: others have pointed out they still use X so I guess the whole point is moot
They should be using custom software and be able to redirect / duplicate the distribution of their news to different or multiple outlets with little more effort than a few lines of code.
Is there any form of API access exposed to third parties anymore on notTwitter?