It’s definitely a generalisation! I was careful to say “many”, not “all” though. I’m not saying all gamers are socially awkward potential sexual predators, just that a proportion of the very very good ones probably are.
Don’t they have to play basically all day, every day, to get that good? Starting as pre teens? How do they have time to do the sort of “normal” social interacting, in real life, that is necessary to grow and develop as a well balanced person? Plus - as @Wumminator said - the whole gaming culture does seem to tend towards being problematic, in terms of attitudes to women. Which you could see with the gamergate stuff. That’s a combination which was always going to have the potential for something like this to happen.
It’s interesting because on one hand you have avid gamers who turn out normal and on the other you have a large section, usually the types who never leave their house or have any kind of social life because they play 24/7 and go to such tournaments, who grow up to be weird, anti-social, extreme incel sexual deviant misogynistic school-shooter types who frequent forums such as 4Chan.