The guy was sitting in his car, not driving. So, what difference would getting cut off really make?
When I was in Portland 20 years ago, I found it very annoying that few cars in Portland would signal.
I think it is better now, but still, one can't count on it.
I suppose I've gotten lazy. I'll signal if it actually makes a difference for traffic flow, or my safety. But, don't bother if it doesn't seem to have much impact.