I've known a few trans non-binary gender folks, including a longtime friend of the family. I don't recall the issue of fairness in sports competitions ever coming up, but back then when they were undergoing the process their main objective to was be as happy as possible in their lives.
From a regulatory perspective it's not always easy to identify a prospective competitor or participant by biological identity. Some folks would nowadays be generally classified as intersex. A friend years ago identified himself as hermaphrodite and biologically male, but had many female biological characteristics. He wouldn't have been athletic enough for his nominally male biologically identity to make him competitive against fit and skilled women his age and size. Being biologically male doesn't always confer physical advantages.
I've known a few other folks like that so it must be more common than many of us realize -- or maybe the stereotype is true that there are more non-binary folks in the arts. That's where I've met most non-binary folks.
CliffordK's proposal may be the least bad of a set of difficult choices for classifying athletes as fairly as possible: one class for those who identify biologically as women and meet some testable standards set by whichever organization is brave and foolish enough to try; and an open class for anyone and everyone who chooses to participate without regard to physical factors -- other than weight classes for martial arts/combat sports.
They could also consider physical performance testing as part of a seeding process, so that competitors will be matched against comparable participants regardless of gender. Again, there's already a somewhat comparable precedent in high school and college sports divisions. Some family and friends are big fans of their Texas high school football teams, but they're realistic enough to know their favorite 2A, 3A and 4A teams won't be turning out many, if any, pros or elite level college players.
While such an arrangement may be interpreted as offensive and condescending to some, that's the nature of physical competition. It's long been accepted in martial arts. It hasn't affected fans who enjoy good competitions regardless of size, strength or even skill. A notable example was the Micky Ward vs Arturo Gatti trilogy. Neither would ever be in any credible top ten list but they were sufficiently skilled and competitive enough to make for three exciting and popular bouts.
And there are competitions in which McKinnon would have little or no advantage, and possibly be disadvantaged by size: for example, a climbing stage against someone like Emma Pooley.