I think it’s because they want to protect against legal challenges by following the science. The science says that it’s male puberty that gives male swimmers an advantage over female swimmers. An advantage that cannot be overcome, however hard the women train.
This allows them to create a special category for ‘people who don’t have the male puberty advantage’, which they’re going to call … ooh, I dunno, why don’t we call it the ‘women’s category’. Nice name.
However, if someone with XY chromosomes hasn’t gone through male puberty, they don’t have the puberty advantage either - which would leave FINA open to a challenge if they defined woman as ‘XX chromosome’.
Therefore, they’ve defined male puberty as stringently as they can.