Where exactly are you seeing this? The NyX midnight Gown states no such thing. Are you maybe confusing the requirements that are listed? For base figures it says: La Femme Base Figure - Included in Poser Pro 11 which means the La Femme Base Figure is included in Poser Pro 11 and the NyX midnight Gown does not require you to purchase La Femme to use if you don't want the Pro version of La Femme.

You can have a look at the amazing vendor content that is included here. There is of course much more in the legacy content as well but this was the most recently added. Hope this helps :)