It's the Y chromosome that triggers them.
edit: alleged/unpublished ... she failed some gender verification tests of IBA that disqualified her there, but met the IOC's criteria. It is what it is. They might keep or change the eligibility rules in the future, and that will continue to be IOC's decision, much as it is IBA's.
https://www.bbc.com/news/articles/crlr8gp813ko seems to have a good explanation. In short, it's complicated, and the IOC drew their lines.