That is a very good question that I have wondered about myself. It's difficult to measure prey consumption rates without allowing any olfactory cue to enter the water, which is why that wasn't included in this study (I would have loved to include it!).
I think that probably the best way to answer your question would be to run simulations and see how results would compare to visual only, olfactory only and combined cues. Visual cues have been investigated before and we have an idea of how they work. With olfactory cues in a still environment, there's indications that detection distance depends on prey group size and that it takes clever olfactory organs/receptors to figure out where the cue is coming from. In terms of simulating a stickleback, I'd assume it didn't get directional cues in still water and purely increased foraging effort in areas where it could detect prey. Larger groups would therefore results in larger detection zones, which sticklebacks would be more likely to encounter and then increase foraging effort in that specific location. Smaller groups would be more likely to be bypassed as the stickleback would be lucky to find itself within the olfactory detection zone of those prey. But this is me guessing. More data on exactly how sticklebacks use olfactory cues would be nice. I think there's probably an interaction happening between some sort of olfactory detection threshold and satiation when encountering large prey groups. Perhaps the most dangerous group size (for prey) would be large enough to be easily detected, and too small to survive detection.
Am I just saying words and not answering your question? I sometimes do that.