Soulkey's vision box did switch to the overlord for a second. He should have seen it. But chances are he jumped vision box to the overlord, but was looking at the minimap, not at the screen itself. Or he wanted to hide his knowledge of that expo from Effort for reasons, and it backfired. Tough to call but odds are in favor of him having missed it.
He probably just didnt see it like human error
You can completely see his reaction during the zergling scout near the end of the game where he clearly didnt know there was a base there