pq safe is only an issue for public key cryptography. Symmetric key encryption (preferably with 256bits or more entropy) is quantum safe from get go. Pre shared symmetric keys (with enough entropy) is the trick WireGuard uses for its pq safety (I guess, the same for Mullvad?).
True. Btw, Signal didn’t get here first. Kyber, the pqkem Signal is proposing to use, is already being standardized since June by NIST for pq safe cryptography. Cloudflare and Chrome (v116) started supporting Kyber for kex in TLS (ref) barely a month later.