He didn't say that bus riders are less intelligent. If you talk to a random person they are pretty dumb, and you find random people at busses. There isn't more to it than that.
Personally I experienced the average person when I did my military duty. Most people were forced to attend so you got a good slice of what society looks like. And yes, the average person there wasn't that bright, the discussions I heard there was the dumbest I've ever experienced.