My point exactly ~ real musical traditions have always been passed down by ear, but the "evidence" you offer to support the claim that no one ever sang in harmony is all based upon written history and records of written music.

I realize that I can't prove that people understood, sang, and played harmony though the ages, and certainly before the delopment of musical notation. That's just my opinion; and while I truly believe this option is absolutely commonsensical, there's no way I can change the mind of someone whose opinion is different.

But, conversely, it is just as impossible (perhaps moreso, if there can be degrees of impossibility) to prove that human beings were somehow incapable of conceiving musical harmonies during any past era.