I'm not certain whether or not it was effectively debunked, but there's a lot of personal belief and interest around the idea that one's gender is what one says it is and no amount of anatomical measurement (genital length, count of brain neurons or genetic flag) is permitted to say whether or not you are what you deem yourself to be.

If such a test did exist, then what's to be said of trans people whose gender expression doesn't line up with the test? What's to be said of cis people in the same boat? Are they trans whether they like it or not? Are they living a lie?

It's a sticky topic either way.