There are very different kinds of “smartness”. The currently favored (by most modern societies) kind weighs logical and structured thinking very heavily, that doesnt mean someone “isnt smart” if they dont possess those qualities. Also, chess does not even rely on the commonly referred to “smartness” that much. The most important aspects are long term memory ( for remembering situations from games and acting accordingly)and thousands of hours of practice.
Good question. I think it has to do with empathy. When arguing on the internet, you dont have an actual person in mind that you are talking to. Also, anonymity gives you safety. You dont have to worry about not hurting someone because it wont have adverse effects on the relationships with people around you, aka your tribe. This was essential for survival some time ago and sits deep within our subconcious.