This.
IIRC, the Horse Armor served as kind of a "proof of concept" test to make sure the infrastructure was working properly - uploading DLC to the marketplace, making sure it installed properly on the game, and so on. Easier for them to do that with a simple and cheap cosmetic item than test the pipeline with a full expansion or something. It ended up being the butt of a lot of jokes, but I'd bet from Bethesda's stand-point it was quite the success - it proved they could put out DLC and that they had best practices in place to do so, and I think I even remember an internet article a while back about how six years after Oblivion released people were still buying Horse Armor from the marketplace...
Occam's Razor would suggest this the more likely reasoning, I think.
With all the settlement-building and crafting, you basically have everything you'd put into a house DLC (and more) already included in the base game. I'd actually be surprised if they even went the Hearthfire route. So this whole hypothetical is itself based on a hypothetical that I, at least, don't find terribly likely.
But to assume all of that ends up being true, and Bethesda just really has it's heart set on this housing DLC - I don't see the strategy linking that to a Season Pass, either. If you just have to release a DLC that from the get-go you know before the game is even released or you've released any details about DLC content people aren't going to buy - how is essentially giving it away for free to anyone who bought a Season Pass a viable or profitable option? What does Bethesda gain from doing that? If this housing DLC is so unpopular, then it's not going to prompt more people to buy a Season Pass, and if it's not going to sell on it's own then you're not going to make more profit when customers only download it because they already have a Season Pass.
So, hey - it's all theories. But to break this down, this situation hinges on a hypothetical that itself has little evidence to back it; and even if those two hypotheticals proved true Bethesda has nothing to gain from employing that strategy.