Actually, it is completely true. The TPM threat model has historically focused on software-based threats and physical attacks against the TPM chip itself - crucially NOT the communications between the chip and the CPU. In the over 20 year history of discrete TPMs, they are largely completely vulnerable to interposer (MITM) attacks and only within the last few years is it being addressed by vendors. Endorsement keys don’t matter because the TPM still has to trust the PCR commands sent to it by the CPU. An interposer can replace tampered PCR values with trusted values and the TPM would have no idea.

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...