When I started working on HyperRogue, I expected it to be a "gimmick or tech demo" as you say. But after implementing the basic roguelike gameplay, it felt surprisingly good, better than what I had actually planned. So released it, and other people liked it too.

So it is not surprising that people expect it to be a gimmick (given that I did expect that hyperbolic geometry would make basic roguelike gameplay feel better myself). Some people do not get it on their first try, but love it on their second try.