look down at vs. look down on

As with so many questions like these, the answer is “it depends.” If you are standing on a hill, you can look down on or look down at a village in the valley. This ngram viewer shows that both look down at the scene and look down on the scene are used, although look down on is more common.

But in a figurative rather than a literal sense, if you look down on someone, you are thinking less of that person than of yourself. To use look down at someone to mean this is very rare at best; if you look down at someone, you’re generally doing it in the literal sense of, say, looking down the stairs at someone standing in the hall.

The example you give is the only time I’ve ever seen at used in this way. An ngram viewer of look down at/on the idea has no examples using at, and quite a few examples using on. This gives me the idea that as an editor, I would correct the passage from at to on.

Now, there’s an idiomatic expression look down one’s nose at something, which has a similar meaning to look down on. Maybe that’s where this comes from.

Doing some more research, I see your excerpt comes from a book authored by one Shankar Vedantam. Perhaps this is a peculiarity of Indian English speakers, but it is not otherwise standard.