Aha. If you click 'Uh' exactly on its text, it works. If you click even one pixel away from the actual text, you get the chooser with not enough options.
First-time StreetComplete user here. I just answered a few of these "street has a sidewalk" questions myself. Upon first clicking on one of them, I was able to answer this question for each side of the street individually. The visual selection that you mentioned only applies to one side.
I can tell for sure the current version of StreetComplete for Android does allow specifying that only one side of the road has a side walk. I solved a few of these quests just last week. No idea what's the state on iOS though.