They also take into account the state of the machine you're working on. So locked bootloader and probably a client cert in TPM-like component, plus "device health". Client certs alone are good for authentication (don't work in HTTP/2 though) but they want to reach even better target - no malicious software running on your computer.
That's from reading old papers, I don't know if anything changed now.
That's from reading old papers, I don't know if anything changed now.