It is correct, the measurement command to the TPM is not encrypted. So with MITM you can record the boot measurements, then reset and replay to any step of the boot process. Secrets locked to particular stages of boot are then exposed.

There is guidance on "Active" attacks [1], which is to set up your TPM secrets so they additionally require a signature from a secret stored securely on the CPU. But that only addresses secret storage, and does nothing about the compromised measurements. I also don't know what would be capable of providing the CPU secret for x86 processors besides... an embedded/firmware TPM.

[1] https://trustedcomputinggroup.org/wp-content/uploads/TCG_-CP...