My thought exactly. Then why an op-amp (with gain) and not a comparitor?
On the other hand:
The slotted photo detector probably has a very wide gain range. AND With room light variations you might need to have this feedback that I see.
With feedback this thing balances at the same point no matter what is happening. (room light, temp, part to part variations)
I am sure there are circuits I designed, and people are looking at them and thinking "What the F&#* was he thinking." It probably solves a problem we might not see from our position.
Also: The "designer" has hours to get this running and now for the next 3 years we can thing of better ways. (that might not be better) lol