In theory it is covered by s159 of the Equality Act on positive action measures. This allows an employer to encourage applications from an under-represented protected characteristic.
The problem is that those "who identify as female" is not a protected characteristic and, as already said, it's confusing language as we don't know if they mean women, or men who identify as women.
If they meant women, they should just say it. It's a defined protected characteristic and, as per para 36 in the For Women Scotland case, provisions in favour of women by definition exclude those who are biologically male.
The other relevant protected characteristic is gender reassignment, which they could have legitimately chosen, but again it needed to be clearly specified. And reassignment is the common factor here, not the sex of the transsexual person, so the employer can't even restrict it to just transwomen or transmen, it has to be both.
But what they definitely can't do is confuse or conflate the protected characteristics by suggesting "woman" also includes those born male with the protected characteristic of gender reassignment (ie transwomen) - which was the ruling in the FWS case.
And just to add, if they said the job would be given to a woman (and did not evoke the genuine occupational requirement in Schedule 9) then that would be positive discrimination, and illegal.