I always think these open letters are a bit unfair in industries that are built on back-biting/back-scratching. Publishing, Arts and Academia are rife with people desperate to cancel one another because they sent on this open letter and that Bad Bad Person didn't sign it.
Any field that relies on word of mouth and personal bonds to get you that lecture, book contract, play staging, TV gig, etc, is going to also be full of people who need to swallow their true beliefs so they can continue to put food on the table. Grim, but true.
The letter itself is total nonsense, as usual; the trans community is extremely vulnerable and marginalised but also feature heavily in the Olivier Awards, the Booker Prize, and the T.S. Eliot Prize, and are explicitly supported by Equity (and others?) who have never been so out and vocal about rights for women or the disabled community. Ahhhh, that women, girls, and disabled creatives and academics could ever dream of such support...