Biological sex for the purpose of the EA 2010 is (and always has been) based on the sex at birth, as determined by medical staff.
Where someone is born with a DSD, doctors will do further tests to determine the appropriate sex to register the baby as. As the NHS link posted above explains, this further testing usually confirms the correct sex within 42 days (i.e. the standard deadline for registering a birth).
So every person with a DSD knows what their biological sex is for the purpose of the EA 2010, as they can refer to their own birth certificate.
Do you really think that anyone born with a DSD will only be thinking about their biological sex for the first time as a result of the SC ruling? Biological sex matters in loads of contexts, so obviously they will have dealt with the same issue before.
If you know someone specific with a DSD, then it’s best to listen to them as to whether they’re confused or not.
If you don’t know anyone with a DSD, but are genuinely concerned or confused, then the NHS website should help you to clarify the situation.
And if you don’t know anyone with a DSD but are just using them as a pawn in the argument for letting biological men back into women-only spaces (or are unwittingly repeating a bad faith argument you’ve heard a transactivist use), then I know there are many people with DSD online who have been quite vocal about the fact they wish the trans community would stop using them and spreading misinformation about them like this.