I highly doubt BBC's social media managers are making long term bets on smaller social media networks eventually investing in activitypub. IRL they probably got little engagement vs the time they were investing so decided to just keep at least one Twitter style feed around so they don't completely abandon the concept of "microblogging"... as the vast, vast bulk of the journalism industry continues to use Twitter and that idea isn't dying any time soon. Having a backup comms platform is deemed valuable enough even if it doesn't drive high business value.
Edit: others have pointed out they still use X so I guess the whole point is moot
At the scale of BBC's social media managers I hope they are not on the x.com website typing in updates and clicking post.
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.
Edit: others have pointed out they still use X so I guess the whole point is moot