Noinheritance with superuser

I have an RDS instance with the below configuration.

create role role1 login noinheritance;
grant rds_superuser to rdsdba;
grant rdsdba to role1

In this case role1 is able to do all administrative operations, without the
need of switching to rdsdba.
Is this expected?
I do not find any document which says that inheritance does not work with
roles which have superuser role granted to it. Note that rdsdba is not a
superuser in itself, but only has a role which is superuser.

--

Thanks in advance.