While this is true, the spirit of the GP is correct. To login you will need both a username/password and the second factor. So losing the key is still relevant. In practice that means people really need to buy (at least) two yubikeys to register with services that allow it. Have one on something that’s always with you (such as keys) and the extra(s) as a backup somewhere secure. Unfortunately this works with most accounts except the big one I want it to... AWS.

Some sites allow you to also register a standard MFA device as a fallback, and further still Google (as an example) allows you to use another device that’s already authenticated to get a one time use code (depending on your account security setup).