It doesn’t matter whether there are differences in each manager’s speed and information — what matters is the difference between when they have a normal amount of coffee and more than a normal amount of coffee.

The argument only deals with what happens when managers drink more coffee than usual. It could be the case that drinking less coffee than usual has a bigger negative impact than drinking more coffee — the conclusion still holds as long as the latter has any negative impact.

It’s not just about the existence of other factors, it’s also about whether drinking more coffee than usual impacts them, and by how much. It might be that communication is a more important factor, but not impacted by coffee. The conclusion could still be true.

If drinking more coffee had some positive impact on management ability that outweighed its negative impact on integrating information, then drinking more coffee than usual would not impair overall management ability. Thus, (D) is necessary.

It doesn’t matter if there is some other factor that has a bigger influence on speed, because it wouldn’t change the size of the impact that coffee has. We just care about how much coffee impacts speed, not how it stacks up with other factors that impact speed.