This is a peculiar part of his ruling. Elsewhere he accepts the following argument made by the defense:
That, she argued, is discrimination on grounds of sex, not gender reassignment: the Defendants’ rules do not involve any discrimination on grounds of gender reassignment, as they do not prevent a trans man from taking part in a women’s competition.
If you were to apply the same reasoning you would say that:
Encouraging greater female participation is a clearly legitimate aim. I do not accept, however, that (were it not for the need to achieve fairness) excluding trans women men from female competitions would be a proportionate means of achieving it.
But it's hard to understand why it would not be proportional to exclude a certain type of men, transwomen, simply on the grounds that they are men. Perhaps he's splitting hairs and that's really what he meant, i.e. it's not the trans-ness but the maleness. But obviously if a Pete the Plumber argument were put to him it couldn't be denied that any participatory argument that applies to men would also apply to transwomen.
Adding, he also strongly discounted the possibility that any interpretation of the SC judgement can be found in 'incompatible' to the ECtHR. It very welcome to get such a strong statement in a judgement so soon:
120 The question of incompatibility was not directly addressed in FWS. It seems to me that this was because none of the participants thought it arguable, and not because it was overlooked.
...
122 Given that view, which was supported by the detailed reasons given at paragraphs [249] to [263], I think it is inconceivable that the Supreme Court would have accepted that the EA 2010 as interpreted by it was incompatible with the ECHR.
123 For those reasons I consider it extremely unlikely that any higher court would grant the declaration of incompatibility which the Claimant now wishes to seek. The position is different from that in Kay, where a binding domestic authority conflicted with a subsequent ECtHR decision, creating an obvious need for reconsideration. Here, the Supreme Court has only just considered the
matter, and there is no subsequent ECtHR decision.