It could be burned into the CPU die by blowing fuses, or stored in a tiny bit of on-die flash, or stored encrypted in SPI flash, encrypted with a factory secret key burned into the CPU at manufacture.

But more generally, you don’t need a long term key to prevent sniffing attacks like this; Diffie-Hellman is a thing. Doing an unauthenticated DH would make this attack harder and slower (active MitM probably requires removing the TPM chip from the board) but would not prevent it.