I saw this too and I had the same questions, actually. Personally I think this is a custom dollhouse in a custom made box. These boxes can be made by customizers and they have been made before for ooak dolls aswell.
The thing that makes me feel it's not legit is the furniture used. It's just shaped like furniture, and with MH I've come to expect certain ornaments, or coffin shapes and stuff like that. These furniture pieces have nothing monster-y about it. Of course I could be wrong.
I think it is legit there seems to be a price sticker on it. Also there is a second one right next to it if you look close you can see a second box it looks like the first one in the corner. It looks like it might be made by fashion angels or one of Mattel's other partner companies.
It looks legit to me, but who the heck knows with Mattel!
it could be legit. the funny thing is last Christmas I bought my daughter a coffin dollhouse for her MH dolls from Costco here in Texas. It wasnt a MH product, but it IS the exact same dollhouse shape/features/etc as that one. They may have licensed MH since then and repainted it, but I find it funny that I had already got that same dollhouse for MH and now they are marketing it for that line specifically.