> You will notice though that the private keyfile held on disk uses a .tpm extension this time. This is where an important caveat of the TPM method comes in – Unlike the YubiKey approach, the private key material is actually being stored on disk here. This is because TPM devices do not natively support enrolling SSH keys. So what is happening is that the private key material is being stored on disk in encrypted form, and the TPM is being used to decrypt it.

While this is true, you can create static hierarchy keys that would not resolve into separate key files for the TPM. The issue here is that anyone with access to your TPM can recreate the keys, unless you set a owner password.

https://man.archlinux.org/man/ssh-tpm-agent.1#Hierarchy_keys