"Firstly, sure there are. I haven't heard anything about Aztec, but certainly Greek and Norse. Not to mention adherents of American Indian myths."
Living people who claim to be adherents of ancient Greek and Norse religions are not. They are whackos who have invented new cults loosely based on what we know of those religions, solely as an act of rebellion against Christianty. They are not legitimate adherents of legitimate religions.
The dirty little secret about American Indian religions is that the Christian missionaries who sought to eradicate these religions were succcessful. What exists today is a hodge-podge of half-remembered stories, patched together with new accretions, that Indians in the second half of the 20th century used to throw up their own rebellion against Christianity. These are legitimate religions in the same way that the Unibomber's rantings are.
Peyote? Used solely by shamans south of the Rio Grande. Unknown north of the Rio Roja. Sweat lodge? Not a religious ceremony, pre 20th century.
"Secondly, I don't see how the question would be moot even if there were no living adherents. Either something is a myth or it is not."
It is moot because the Constitution deals with the government of the living. It protects the rights of living citizens, not the long-dead. The question here is not whether anything is or is not a myth; the question is whether using public funds and the resources of a public institution to label something a myth infringes on the free exercise of religion by living US citizens.
I expect that you will next argue that if freedom of religion extends to Christianity that it also extends to any nutso crap that any looney-toon whacko cares to invent.
It doesn't.