So hibernate is somewhat unreliable and prone to data loss, image you hibernate after having installed a new kernel, so the decision was made to disable it due to that IIRC, independent of secure boot.

With secure boot and lockdown, hibernate is no longer possible on an alternative reason: We need to ensure that the kernel memory has not been tampered with. If you hibernate, you could then go and modify the memory in the swap and bypass the lock down security guarantees.

To address that you'd need to authenticate the swap using the TPM somehow, but I don't know enough about TPMs to know if that's feasible. Usually people would seal some crypto key against the TPM but here it's somewhat the opposite way around.