You are right presharing is a requirement, unless you hash the keys used to encrypt the secret into the secret itself, but that can only be prooven later on a channel where the same MITM is not present.

Work in progress, that said presharing solve(d/s) enough for the world to dump DNS and HTTPS in a bin and light it on fire now, because nobody has the power to implement all the MITM needed if everyone "makes their own crypto" on top of allready shared secrets!

Circular arguments, wishful thinking and all...