This (and Signal's solution as well) does not protect against active MITM attackers with quantum computers. They would need to incorporate post-quantum signatures into it as well.

The reason why it is missing (but seemingly planned in the future) is because it is not as critical as this change. This change prevents attackers from recording conversations now and decrypting them when (in the next ?? years/decades) they get access to an actually powerful quantum computer. On the other hand, you can do MITM only after you factorized RSA key (or solved discrete log).

The additional reason I presume is that this typically requires a change to the whole public key infrastructure (certificates, OCSP, etc.) which is a lot of additional work.