Yeah, that's right. The game will transfer all sims in the subhoods into the main neighborhood package file and merge with the already existing townies/NPCs family and/or create new strays/garden club members/tourists families. But this is also what happens if you also attach a university subhood or a downtown subhood. So nothing new really.

The Pets, Weather and Exotic Destinations stealth hoods are just there as dummy neighborhoods and cannot be accessed in-game. But I think EA made those stealth hoods so that the game can recognize whether or not the strays, garden club members, etc. have already been added to the neighborhood. This will also prevent them from accidentally being added again for the second time.