No, signing is done with asymmetric keys. Someone sign with their private key so you can verify it with their public key, basically. Symmetric keys are used to encrypt the code for updates, generally to prevent unintentional leak of secrets or just to prevent reverse engineering.

I'm not in the jailbreaking space, but I guess that what it means here is that some people may find flaws in the code and leverage them to potentially bypass the signature verification, although this might prove very difficult to do. It might also help emulating the PS5 behavior which in turns could lead to some modding/cheating for some games. Who knows ?