Just I would guess that, at the moment, the number of Mumsnet users overall vs the number of active posters on FWR who are concerned about women's rights being eroded by trans-rights activists probably is a minority.
Because:
a) Lots of users, like me mostly, are dormant, or 'lurkers'. I read along, I don't post. You couldn't count me in your % right now if you wanted to judge 'how many people like grey walls on Mumsnet'' or 'do you think owning a toilet brush is beyond the pale' assumptions. But believe me, I have opinions on both! 
b) By doing this, she's talking about the issue, raising it, so there's a strong possiblity that more people will catch on to what's actually happening right now, and be more active.
c) I'm only starting to be more active because I've seen so many comments elsewhere of 'oh, it's only a small group of radical people', 'oh someone's whipping up hysteria in Mumsnet'. I've realised I need to add my voice and support those who are already speaking out. The flag of 'minority' brought me out of my cave - it may do for others too!