More of a person with IETF participation experience than as a cryptographer (I enjoy watching numbers dance but am not a choreographer):

This ( https://datatracker.ietf.org/doc/draft-ietf-tls-mlkem/ ) is a document describing how to use the ML-KEM algorithm with TLS 1.3 in an interoperable manner.

It does not preclude other post-quantum algorithms from being described for use with TLS 1.3. It also does not preclude hybrid approaches from being used with TLS 1.3.

It is however a document scoped so it cannot be expanded to include either of those things. Work to define interoperable use of other algorithms, including hybrid algorithms, would be in other documents.

There is no MTI (mandatory-to-implement) once these are documented from the IETF directly, but there could be market and regulatory pressures.

My suspicion is that this is bleed-out from a larger (and uglier) fight in the sister organization, the IRTF. There, the crypto forum research group (CFRG) has been having discussions on KEMs which have gotten significantly more heated.

A person with concern that there may be weaknesses in a post quantum technique may want a hybrid option to provide additional security. They may then be concerned that standardization of non-hybrid options would discourage hybrid usage, where hybrid is not yet standardized and would likely be standardized later (or not at all).

The pressure now with post quantum is to create key negotiation algorithms are not vulnerable to theoretical post quantum computer attack. This is because of the risk of potentially valuable encrypted traffic being logged now in the hopes that it could later be targeted by a post-quantum computer.

Non-negotiated encrypted (e.g. just using a static AES key) is already safe, and signature algorithms can be updated much closer to viable attacks to protect transactional data.