That Starmer fella...

why does a trial have to be double blind if you have baseline data to work from?

Worth stating that Cass says these trials don't need to be double blind. The University of York (rather than Cass) was responsible for saying whether a study was or wasn't included, and they say they included around 60% of the existing papers - though it is fair to say that they only found 2% were of sufficiently high quality. They also say they didn't exclude on the basis of double blinding, which is as we all know not possible when a drug changes something physically obvious:

Blinding is a separate issue. It means that either the patient or the
researcher does not know if the patient is getting an active treatment
or a ‘control’ (which might be another treatment or a placebo).
Patients cannot be blinded as to whether or not they are receiving
puberty blockers or masculinising / feminising hormones, because the
effects would rapidly become obvious. Good RCTs can be conducted
without blinding.

The University of York’s systematic review search did not identify any
RCTs, blinded or otherwise, but many other studies were included. Most
of the studies included were called ‘cohort studies’. Well-designed
and executed high quality cohort studies are used in other areas of
medicine, and the bar was not set higher for this review; even so the
quality of the studies was mostly only assessed as moderate.

(source: https://cass.independent-review.uk/home/publications/final-report/final-report-faqs/)