I think Heine forgot the word 'section' after 'rear fender'. That would make the sentence I quoted make more sense:
"The front section of each fender shields the tire and reduces the wind resistance, while the rear fender section increases the aerodynamic drag. The two effects cancel each other."
It's quite obvious I'd say, looking at the first part of the sentence.
So it's not the rear fender that increases the drag but the rear part of each fender, in contrast to the front part of each fender which shields the tire. Since the front fender has hardly any front section shielding the tire, the front fender on its own would have a negative effect on aerodynamics, in contrast to the rear fender which is almost completely shielded and must have a positive effect to cancel out the negative effect of the front fender. Or in other words: usage of a rear fender would make your bike more aerodynamic. This is just theory though, Heine would be able to confirm or correct this based on his tests, I assume.
Either way, the point is that well fitted fenders have a negligible effect on the total aerodynamics of a bike with rider.