How about the limited number of exceptions to the XX just play in any of the open divisions? Or make them all open divisions? An XX chromosome only division isn't much of an XX chromosome only division if you allow exceptions to XX chromosomes.
It never claimed to be an XX-only division.
It's a "female" division, with "female" defined, for these limited purposes, as XX and/or a few other situations. They're not defining "female" for the dictionary or the overall population, just for this usage. Which works well enough, since 99% of the participants will be considered female by virtually any definition.
Just as they defined "masters" as players over 40 years old, which may not be the definition of "master" that anyone else uses. Or define "throw", for that matter.
The PDGA could define it as XX-only, of course. Among the reasons not to are that by adopting IOC-like standards, they don't have to devote a lot of time and effort and debate to what has been a non-issue.