Seeing as they held their conference in Florida, I suppose verifying this would be pretty much impossible.
In states with formalized self-id, like California, one could require ID cards to verify gender and opportunistic ploys like this would require more hurdles.
Not surprisingly, you actually have this backwards.
In Florida, to legally change your gender on your driver's license, you need a formal statement from a physician who is actually providing transition treatment. That's not self-ID, that is a real hurdle, and there's more than just your own word about your status.
In California, you can change your gender on your driver's license with only a self declaration. That's a very tiny hurdle, and it's only your own word, with no mechanism to challenge it. Doing so is basically free.
I have no idea on the legality of this generally, or how a public conference can discriminate in this way to start with, but this seems largely a problem that results from the ambiguity of having the state adopt of position that denies the existence of trans and non-binary people.
What state? Florida? Nope. Florida doesn't deny the existence of trans people. They just don't use self ID. That the conference organizers went with self ID isn't the fault of Florida.
Real problem in the US as state laws vary wildly, but one can imagine that this would be a non-issue in Canada, for example, as people's declared identity is verifiable.
Only in the sense that you can verify what they declared. But that's just circular. Nothing other than a declaration is required. So if what you're trying to do is figure out whether a declaration of identity is a lie, verifying that they declared that identity can't achieve that.
And here we see the failure of your position exposed most plainly. There are two fundamental facts at play which were always obvious but you have desperately tried to ignore:
1) If you create an incentive to lie about your gender, people will lie about their gender.
2) Under self ID, there is no mechanism to detect or prevent such lies.
The conference organizers created such an incentive to lie, and by their own choice could not detect or prevent such lies. The results were entirely predictable, and entirely their own fault.