Either <connect:allow> or <connect:deny> is required or connect class is not suitable about inspircd-docs HOT 1 CLOSED

Good catch! You are correct that one of allow or deny are required in a connect class for typical user connection matching. In the referenced example, an allow="*" should be added.
I'll dive a little deeper, for fun, should look at adding this to the docs tbh. Actual requirement is one of name, allow, or deny. These define the type for the connect class. An allow or deny type can have a defined name or be nameless and the server will assign it a name. A named type does not contain an allow or deny value and will not match during the core user connect cycle. These can be used to define a parent (or base) class for various purposes and modules can force a user into a named class.

from inspircd-docs.