The point of the women's leagues is so they don't have to compete against men because women are biologically unable to compete at the same high level as men. If a biological woman decides not to identify as such why would she be allowed to continue in that league? Isn't that the point of allowing mtf transgenders to compete against women if they identify as such? It's logically consistent.
Quinn identifies as non-binary (they/them). So while they don't identify as a women, physically they have the same body as "a biological woman".
MtF players are a different situation, because they frequently go through HRT. The body changes drastically because of the new hormones and a lot of strength is lost.
If Quinn was FtM and going through HRT, then this would also be a different situation. Their body would change drastically and gain a lot of strength. But this isn't the case.