As I understand it, Merry, it's about a specific set of criteria in law called the "Grainger" criteria - it's not about whether Maya's views constitute a lay-person's understanding of "belief" but whether they meet the standard required for a belief that meets the standards such that the believer is protected from discrimination under law for holding that belief.
So for example, someone could not be sacked from McDonalds for expressing the belief that "Jesus Christ is the only risen Lord" on their social media feed, but could (presumably after the relevant disciplinary procedures) be sacked for expressing the belief that "McDonald's burgers taste of shit" on their social media feed.
Four out of five criteria were met, but in the judge's opinion, one was not, specifically the criterion that for a belief to be protected, it mustn't impinge on the dignity of other people.
This, in the judge's view, put Maya's belief that sexual dimorphism is biologically established fact (or as near to as science is capable of delivering) into the "McDonald's burgers taste like shit" category rather than the "Jesus Christ is the Risen Lord" category.
Batshit crazy, IMO, but that as I understand it is where the legal basis came from. Whether it can successfully be appealed is another matter (having read the judgement, I'd say there's quite a few places where you could mount an appeal).