Some of those resolutions get pretty low according to Digital Foundry, but I didn't seem to notice while playing through the demo. I wonder if it spends the vast majority of time at the 720p mark, and just on rare occasions dips below that (like the same frame that an explosion occurs or something like that).
I'll watch the actual video later when I'm not at work.
I'm trying to think if there could be a logical reason to release the demo at a lower resolution than the final product, but I can't come up with anything I find reasonable. Sure, Nintendo would want their demo to have a low file size, and they could pack in lower resolution textures to meet these demands, but they would still want the game to run at a higher resolution regardless, I would think.