Occam's Razor would suggest this the more likely reasoning, I think. 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. Easier for them to do that with a simple and cheap cosmetic item than test the pipeline with a full expansion or something. 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.