The less windspeed comment does make sense. As the water moves at 3 knots in the same direction as the wind, it's just like having 3 knots less wind. Consider your car on a conveyor belt. If the wind was 10mph and you are driving into it at 10 mph, you would normally experience 20mph of apparent wind. If your car was now on a conveyor belt that was moving 10mph in the opposite direction (so your car is now not making any forward progress), the apparent wind would be only 10.

It doesn't matter foiling or not - the speed vector of the water current that is parallel to the wind directly subtracts (or adds!) to the wind speed.