The mk_oracle-Plugin opens Connections only with a running listener. If the oracle_instance is OK the listener is ok as well, because the check was done with a connection over the listener.
Checking the listener with check-oracle-health --tnsping sends pings to the listener port. Just use a tcp check instead of check-oracle-heath - much easier to configure but not really needed, because the error in the Instance Check is easy to understand, when no listener is running.
That’s the reason why there is no dedicated check for a listener in mk_oracle - this is normaly not needed.