Indie RPGs are postmodern because and only because all role-playing games are post-modern.
1) They're postmodern in time. Just like I can't write a modernist poem, I can't write a modernist role-playing game. If you're willing to stick the invention of the RPG in the 70s, unless we find a secret cache of GM notes by Ezra Pound, no one was around to write a modernist role-playing game.
2) They're post-modern in form. They are not stories, they are a means by which the audience creates stories for itself.