This advice is not very good. Probably the best thing in this list is using the “AllowUsers” directive (which makes limiting root access a moot point) and using

This advice is not very good. Probably the best thing in this list is using the “AllowUsers” directive (which makes limiting root access a moot point) and using a good, strong key. There’s not much benefit to using a cert over a password-protected ssh private key.