Poking around it seems like pre shared keys are used for the secure stuff, so no public keys, no rsa. It isn't that signal isn't state of the art, it just makes compromises for usability.

Edit: I didn't state something perhaps I should have. Symmetric key is considered more secure because public key is more complicated so more room for side channel mistakes, and the computation needes to break public keys doesn't scale as fast with key size. I am not an expert but that is what I've read.