But isn't that part of the whole Public Domain thing?
Intellectual property law in licensed works is a weird thing. Red Sonja in the comics is basically 90% a Roy Thomas creation with very minor connections to the work of REH. If she'd been called Black Sally and debuted in something other than a Conan comic, she'd be a 100% owned Marvel character.
BUT...Red Sonja isn't actually owned by the REH folks either. There's an independent company that owns Red Sonja that is separate from the people who hold the rights to Conan, Kull, etc.
BUT...that independent company licenses the rights to use Hyboria as a setting for Red Sonja stories from the REH people.