An SSH key is one factor. There are various methods for protecting the key with additional factors, but none of the git hosts provide a way to require those additional factors. So as an org owner you're left either trusting every one of your users not to get sloppy with keys, or installing spyware on their computers to make sure they're not using unprotected keys.
How so? An SSH key is a single factor. You could argue that a password-protected private key provides a second factor, but that still falls in the category of "something you know."
> The fact that one time passwords expire and change is what makes them a different factor than a static password.
If you're getting your 2FA code by SMS message or the like, this can be true.
If you're using TOTP (e.g. Google Authenticator), that's just as static as your other passwords. The TOTP code never expires nor changes. What changes is the code you're supposed to send over the wire.
A 60-second TOTP code is a fully deterministic function of a permanent, unchangeable secret. That's why you and the server can agree on what the code should be without needing to communicate beyond setting up the code originally.
This makes it identical to a password from a theoretical perspective. There's really no difference between a TOTP secret that you keep in a TOTP app and haven't memorized, and a password you keep in your password manager and also haven't memorized. Both are "something you know", and nothing else.
You're correct that leaking a temporary code from a single login attempt doesn't compromise the TOTP secret. That is an artifact of the login process, not of whether the mechanism is labeled "2FA" or "password". You can do the same thing while calling the secret a password: https://en.wikipedia.org/wiki/Secure_Remote_Password_protoco...