I think this is a misrepresentation.
Very few Genderists claim biology is fully mutable.
Common claims are:
Sex isn't biology, it's mental. So people can have the wrong sex for their body. They don't change sex, they just correct their body.
Sex isn't biology, it's cultural constructs wrongly applied to bodies. So you can be either sex, or no sex, regardless of what body features you have.
Sex is biology but because there is overlap between secondary characteristics like height and strength, and because DSDs and gender medical interventions mean biological features of one sex can appear in the other, means no one can ever really assess someone else's sex so you just need to take their word for it
Saying women are a certain sex is reducing women to baby makers and we don't think that is ok any more so why can't male people be women?
These ideas get treated like a pic n mix, so you quite often find the same person using contradictory arguments at different times, or even at different times in the same conversation.
Yet for all the handwaving, none of them ever explain why, if biological sex isn't important to who is a man or a woman, is it so important that the pre-existing language and provisions that were set up because of biological sex and the impact has on us, that only exist because of biological sex, are nevertheless exactly the right ones that need to be used for the thing that is definitely not biological sex, and that any language or activity or support or social analysis that is based on biological sex can't exist alongside it but has to stop being done altogether.
Because honestly, some of those ideas I would actually be entirely comfortable with if they were just following their own logic and treating Gender as something totally separate to body sex.
Just give this thing that is not sex a different name to the names of the sexes, and let us carry on also having names, rights and recogition of the impact of our bodies and the history of sex and sexism on especially women.