From man sshd_config:

 PermitRootLogin
             Specifies whether root can log in using ssh(1).  The argument must be “yes”, “without-password”, “forced-commands-only”, or “no”.  The default is “yes”.

             If this option is set to “without-password”, password authentication is disabled for root.

             If this option is set to “forced-commands-only”, root login with public key authentication will be allowed, but only if the command option has been specified (which may be useful for tak‐
             ing remote backups even if root login is normally not allowed).  All other authentication methods are disabled for root.

             If this option is set to “no”, root is not allowed to log in.

You should not mistake permitting normal root login and root login through ssh.
As above, authentication via password via key are two different things, and do not affect each other.

If you set PermitRootLogin without-password in your SSH server config, you will be able to log in only via the key and not the password. Hovewer, most server admins do set up a separate account with sudo access and disable root login completely (PermitRootLogin no).