it's not back-formed from postpone?

I wouldn't call it back-formation, but maybe others would. I see on the other words board (protocols be damned!), Faldo has asked after prepone. I think of classic back-formation as something like babysit from babysitter or burgle from burglar. Here, the post of postpone is being replaced by another Latinate preposition-particle thingie, i.e., pre-.

Ceci n'est pas un seing.