The left arm bent up for a left turn on a bike was originally used when cars did not have turn signal lights. Most people these days have either forgotten, were never taught or are ignorant to what it actually means. Left arm straight out pointing left and right arm straight out pointing right is the best method to use when turning.

I do not understand this guy's comment about being vigilant and not wearing a helmet. He is obviously one of the confused masses who thinks wearing a helmet correlates to the skills needed to safely control and ride a bicycle. By this guys logic because he does not wear one he thinks he has to ride better and safer, though obviously form the OP's description of his riding skills he doesn't. And with a helmet he can ride like the OP described him as doing such.