Does the specification recommend using a hybrid? At the level of at least SHOULD if not MUST, in the sense of RFC 2119? I have the opposite impression but haven't checked.

In effect, yes it does.

The Recommended flag is set for X25519MLKEM768. It is not set for any of the pure PQ key exchanges.

https://www.iana.org/assignments/tls-parameters/tls-paramete...