JK, in contrast, knows how far from the existing position this is
I agree with this. The current position and orthodoxy can be read by watching how the Beeb treats it, and at the moment that is still no debate/TWAW. McDonnell has implied that it can be talked about, so now everyone will be checking behind the scenes to see if he meant it. They will all be reassured privately that he didn't mean it, and all will go on as before. There will be no debate, and extreme transactivists will be given the louder voice.
John McDonnell doesn't care about women on this issue one scrap, not at all, and nor does he care about trans identified people, one bit. He realised that it could have an impact on his wider project, and is vote counting. He's just appeasing both sides and hoping to keep that up until an election/leadership challenge.
If you suddenly see more GC balanced items on the BBC that will be a sign that they've been given the go ahead from on high to push back against Trans Media Watch. But they won't be, and we won't.