I think to get back to the core of the case, it is about deception. Not even whether someone claims they are trans or not.
The problem is that the whole of transgender ideology is based on the 'right' of a transitioning person to deceive everyone else. There are clauses in the GRA about when it is legitimate to disclose that someone has a GRC, which means that there are times when it may be necessary to lie in order to not break the law.
In fact, one of the major reasons for the existence of the GRA was so that a transsexual didn't have to out themself to their employer. Or to put it another way, the reason for the GRA was so that a transsexual person could deceive others about their sex, including their employer.
From the LGBT Foundation article:
The conviction then of Kyran Lee leaves us questioning; do trans people actually have the right to hold the details of their trans histories private or must a disclosure happen before engaging in sex with new partners? We know that trans people face stigma, hate crime and violence at a disproportionate rate. We also know that there have been instances where trans people are met with violence and assault when disclosing their trans status before having sex with a new partner, understandably, many trans people choose to not disclose their trans history.
In the cases that I've read about where a trans person didn't disclose that they were not the sex they appeared to be and violence ensued, it was after sexual activity had taken place, and a heterosexual man felt that he had been duped into having sex with another man. The time to disclose is before sexual activity so that the other person can consent, or decide not to go ahead.
Why on earth do they think that the best course of action, if a person might not want to have sex with them if they know the truth, is to withhold the truth?