They realise it. It's the same thing as his suit against 60 Minutes. He would have had to prove damages, of which there were none- it's actually easier to win defamation/libel suits in the uk than the us. But it's a tactic to get them to kowtow to him/settle with him/be beholden to him going forward. Like CBS did, with the giant payment and their recent kid glove treatment of him. The very last thing he and his minions want is to actually end up in court, which would be very damaging to him.
I just hope that the BBC, which unlike CBS is taxpayer funded, decides to hold firm. Although I suspect, in trying to avoid another tariff fiasco, they're keen to keep his ire tamped down, so will issue a grovelling apology. Not to mention that people like Robbie Gibb seem pretty inclined in his direction anyway.