I may not be popular for…
I may not be popular for saying this, but my vote is that this driver is, in fact, stupid. I my sphere of influence, this is common knowledge, and also is not specific to this make and/or model of car. However, with the transition to LED lights, this indicator (pun not intended) will go away since there's not enough resistance in LED bulbs to trigger a difference in the speed in which the turn signal flashes.