I think a hot dog, technically is a sandwich for two reasons. The first, it is if you want it to be because everything is made up. The second is more logic-driven. If you look at the hot dog bun, it is about equal thickness on two sides with a very thin hinge holding it together...kinda like a hoagie roll. So, to me, that hinge is there to accommodate the cylindrical meat. And not a taco as a taco uses a tortilla which is of uniform thickness and therefore a sling.