Let's imagine for one moment that Pride was just for LGB people and Easther and Vernie had said they did not wish to perform at these events because the believe that homosexuality is a sin.
If Louise had reached out to her gay fans and said, "Guys, so sorry Eternal will not be performing at Pride. I would have loved to perform for you guys but unfortunately a couple of band members feel it is not in accordance with their religious beliefs. I am hugely disappointed about this and will always love and support the LGB community."
What would the likely consequences have been?
I suspect there would have been a mild stir on Twitter with people saying things like, "What a shame. Bigotry will not win."
Easther and Vernie might well have been roundly criticised, but that probably would have been all that happened, because as far as I am aware the LGB community does not have form for being aggressive and violent towards people who for whatever reason do not support them.
Not so with trans activism.
It is entirely plausible that Louise's comments might result in, for example, an organised campaign to have Easther and Vernie cancelled, dropped by their agent/publisher/record label/whatever they might have going on these days, reports made to the police alleging a "non hate crime incident", people finding and publishing their addresses online, people standing outside their homes where they live with their children holding placards and shouting aggressively, people sending rape and death threats, and various other intimidation tactics.
These things are plausible because they are all things trans activists have done to women who disagree with them.
If Louise knows that this is how some trans activists choose to behave and she dropped Easther and Vernie in it anyway, that is despicable.
If she doesn't know that this is how some trans activists choose to behave then she needs to better inform herself about this particular political bandwagon before jumping on it. A good start might have been to actually ask Easther and Vernie what it is about the TQ+ community they object to.