Incorrect. In my testing, if no Runas_Spec is specified the command may be run as any user and the -g option may be specified to select any of the target user's groups.

If no Runas_Spec is specified it is the same as (ALL). https://unix.stackexchange.com/a/13053 But if your intent is "execute as root", put (root).

Note that is says in addition. This implies that, without specifying the second Runas_list, the target user's groups (the user's primary group and any supplementary groups to which the user belongs) can still be specified via the -g option. I confirm this on my Fedora 34 system.

This part:

but no -g option may be specified

is incorrect. The -g option may still be specified to select any of the target user's groups (the user's primary group and any supplementary groups to which the user belongs).