I don't know what the topography of houston is like, but here in toronto, a few hundred meters would move you from the bottom of a deep river valley to the top of it. I would imagine they made sure they could get insurance before building and wouldn't have picked any place with a significant risk.
The topography of Houston is that everywhere is a few hundred meters from a flood zone. You are exactly right; the area did not even come closer to flooding during Harvey and is a good 30ft higher than the flood zone OP is referencing.
