All of the tabards for the promo are called “classic,” because they’re just basic, simple, classic designs. Not because they have anything to do with WoW Classic. I think it’s the same way with the Trading Post ones, too. The sepia one even says:
“A simple tabard for many occasions.”
Did you follow the steps in this article? I have no idea if the promo ended or not.
I try and always default to “user error.” So I make posts like this. Hoping someone will say “oh yeah, I did that about a week ago… it’s not obvious, but this is what you’re doing wrong!”