What a load of absolute nonsense. Sex is immutable. You can't suddenly grow a pair of ovaries or mammaries or testicles or a functioning penis just because you feel like it.
Gender is mutable because it is a social construct. Gender can be fluid and non-binary. Sex can't.
But biological women have always been controlled by biological men and deserve the right and the respect to create discrete spaces away from biological men without being bullied by biological men into having these boundaries removed for the gratification of men.
If I were them I'd close down, set up again as a private invitation only site and charge uninvited self-referrals...I dunno...five thousand pounds?... to join. Whatever the cost of their payment to him was.