Mentioned guide is about restriction of access of user with selected role to other roles during new user creation or user editing.

URE Pro does not realize any filters for which roles can be granted as primary or which roles can be granted as others only.

More, WordPress does not realize any special attribute to differ primary and other roles granted to user. There are just a list of roles, PHP array. 1st role in that list is counted as primary role. If you revoke that role from a user, role which was 2nd in the roles list for this user, will become 1st, or by other words – primary. WordPress checks user permissions by simple sum of all capabilities from all roles assigned to a user.