"E hele i uka" seems to be a legitimate answer as well, no?
I agree! Why isn't accepted? It appears to have the same meaning.
We had "E hele i kai" (or I thought we did -- need to double-check), so it seems that "E hele i uka" should work too. I reported it as "My answer should be accepted." Weʻll see. :)
Why not E iho i uka?
That would mean "Go down toward the upland" which would require a very specific geography and usually not make sense.
Pi'i seems to have an expectation of a directional - go up, go down, go inland. Hele seems to be more general - go to the store, go fast, go to the gym.