I'm not sure how it's odd. It seems logical to me precisely because of the chronology you outline.
The article states: "On 16 April the UK Supreme Court ruled that the legal definition of “sex” is based on biological sex assigned at birth." This is incorrect. If this were the case, then Imane Khelif, who was "assigned" female at birth, would be "legally defined as" female in the eyes of the Supreme Court. In fact, as we know, they're male. (My understanding is that they would be seen as such by the SC, too, but IANAL and maybe this is something to be tested in future).
Regardless, I agree that the conflation of DSDs and trans status is cynical and exploitative. That's why accuracy and precision are vital. In this particular case, the misreporting of the SC judgement could be cited to support a male boxing against females. This would be wrong.
I'm arguing against the appropriation by trans ideology of the DSD-specific descriptor "assigned at birth". I see the Guardian's misuse of it as disrespectful and damaging.