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