I don't think it's entirely reverse ferreting. The Observer has always been slightly less awful on this issue than the Guardian. Catherine Bennett has written gender-critical pieces in the past, though her articles tend, in my view, to be quite circumlocutory and less forceful than, say, Janice Turner's, so they have less impact. It was a mistake, for example, to begin this piece with a lot of stuff about the definition of anti-semitism rather than with the specific transphobia argument.
It would have been quite hard for the Observer to ignore what's going on in the Labour Party at the moment, given the extensive coverage it's had elsewhere.
There has to come a point, I think, where both the Guardian and the Observer have to realise which way the wind is blowing. My guess is that the Guardian's own staff have for ages been telling the management that there is "no debate" and that only extremists oppose self-ID. They are now belatedly realising that is not the case.