Yes HSTS (homosexual transsexual) will be a biological condition, but considering that they can't confirm that homosexuality is biological (homosexuality and transsexuality are related to hormonal levels the foetus receives) they can't prove the biological basis for transsexuality without proving that for homosexuality.
Heterosexual transsexual (AGP) is not a biological condition, and that is what the activists fear. This is why clinics used to be able to filter them out from the gender transition services, and campaigning seeks to end asking about the sexual element.
And then we also get the people who self identify (who are not part of either above groups) who use it for various purposes. ROGD are also not any of the above groups, so clearly there is no biological basis there, just social conditioning.