That FB post from Mermaids didn’t actually say very much. No recognition that any child needs a lot more time and freedom to explore their identity than being labelled as trans when in pre-school.
It’s good that the judge has questioned the current orthodoxy, and emphasised the need for professionals to not fall into that trap.
It is very rare for a young child to believe that their gender is different from their sex; and of course they need support, especially in trying to understand why they believe that to be the case. This judgement is a useful reminder to the professionals that they need to balance acceptance and exploration. Child psychology, as a field, is probably one of the most challenging fields to work in. Young children can have a very wide vocabulary, but don’t necessarily have the nuances to even explain things to themselves, let alone other people. (Which also goes for many adults as well.)
If this young boy is trans, he will transition later. All the evidence, especially from the judge who actually heard both parents in the court, shows that the father will support his child.