I have heard arguments that using a password locked ssh key is two factor authentication for the same reason, that is, what you have(the ssh key) and what you know(the password to unlock it)
However I am not convinced, I think it comes down to dependency of factors in the auth scheme, if one factor unlocks another factor there only really is one factor, all factors must independently lock.
The "something you have" shouldn't be easily replicated. If someone could capture your SSH key passphrase and copy the ssh private key then it's not an actual 2FA. a device like a Yubikey won't let you extract the stored keys, so the only way to gain access is to steal the physical key and that makes the attack much harder to do stealthily.
