From https://signal.org/docs/specifications/pqxdh/#passive-quantum-adversaries

PQXDH is designed to prevent “harvest now, decrypt later” attacks by adversaries with access to a quantum computer capable of computing discrete logarithms in curve.

Also:

PQXDH is not designed to provide protection against active quantum attackers.

Basically this makes it pointless to collect any data now with the intent to decrypt it in the future - e.g. the NSA collecting all your encrypted messages to decrypt them all in 5-10 years once they have a capable quantum computer.

It does not protect against an active quantum attacker - of which there are currently none, so work in the field is likely expected to continue.