He's saying that when the wheel arch is closed like stock the air will form a sort of bubble and under-body airflow will treat it just like a "phantom" belly pan. Letting air out of this bubble somewhere else will just direct air into this high drag area, increasing your overall drag.
What comes out, has to go in. And "in", in this case, is a very turbulent area.
Honda's Insight and more notably Ferrari's 250 Testa Rossa have rounded the rear of the front arch to facilitate better flow in this turbulent area. My advice is to install flat wheel covers, block in the front and top of the wheel gap, and leave it alone. Using a rear wheel skirt will help you a bit but rounding out the edge of the arch is probably a waste of time afaik. You could attempt some front wheel skirts but I'd be more concerned about a boat tail at this point. Low hanging fruit.
I hope that helps.
