The whole point of TPM is to enable passing keys so users can still view content (which needs to be decrypted to be shown at all) without being able to access the keys themselves (because it has its own public/private keys and the keys are encrypted using the public keys and the private ones aren't supposed to be visible outside of TPM).

If this exploit gives access to TPM and its private key, then you can use that to decrypt keys as they are sent and then decrypt a copy of the content that gets saved as such (instead of immediately re-encrypted with your monitor's public key so you can't just intercept the signal on the wire, which is also why HDMI has been unwilling to license an open source implementation because those details could reveal flaws and just one flaw could mean all media up to this point is now piratable without an insider leaking anything).