Oh ffs, you don't have to like Richard Dawkins or agree with him on lots of stuff. I happen to think he's a brilliant communicator when it comes to science, but his attitudes on loads of things are extremely disappointing and downright unpleasant.
In this context, that doesn't really matter.
His support is important for two reasons. One is that he's a very high-profile evolutionary biologist, and he understands that sex matters. A lot of the trans activist argument rests on the idea that sex is a spectrum, or sex doesn't really matter, or was invented by Western colonialists, or is less important than gender identity. Obviously this is nonsense but they frequently claim that their view is backed by "science" or "scientists". Quite clearly it isn't.
Second, Dawkins has a high profile in the skeptic community. Hilariously, for people who claim to be sceptical about pseudoscience, they have to a large extent been captured by the gender woo-woo. So the fact that Dawkins has come out and said it's nonsense might give some of them the courage to publicly agree with him.
The point is, we need public figures to speak out. We particularly need well-known scientists to speak out. We need the message to go far and wide, and we need it to reach people who are sitting on the fence or too timid to express a view.