Well, the argument that CB/FF "would be crazy" to reveal his alibi before seeing the prosecution's case dossier is now obsolete. So the next question is, why now? Why has it taken 20 months since the appeal went public for him to offer up an alibi? And why do it through someone like MWT who doesn't exactly have the greatest reputation? Maybe that's exactly why they've used MWT though, specifically because of his past work on Pistorius etc.
It feels almost engineered, a way for FF to claim proof that his client was plausibly elsewhere to put the onus on the prosecution to prove this account is untrue. It seems risky that they are putting this out there without knowing whether HCW already has proof to show otherwise. But maybe that's the point, by revealing his "alibi" now, they may feel it gives his story more credibility at trial than doing it after they've seen what evidence exists against him and then tailoring an alibi to suit.
Perhaps, as has been discussed, there was more than one person was involved. And while CB had used the phone number mentioned in the appeal, he knows it was actually in someone else's hands during that half hour call. And therefore, this is the time period his "alibi" relates to because he knows he was actually somewhere else at that time. Maybe on the other end of the call even.