You are making up stuff.
Skill and pretentiousness are independent variables. Assuming that one is correlated with the other is a sign of poor judgement. I know people that fit would fit in each of the four quadrants {skillful-pretentious, unskilled-pretentious, skillful-humble, unskilled-humble}.
Anecdotally cooks are not usually pretentious - perhaps in your circles or in your city things are different? Personally I've got little time for pretentious people.
> I suspect even they can't tell if melted cheese within a dish started out pre-shredded or not.
I didn't say that. But https://news.ycombinator.com/item?id=43537461 did say that.
You can deny the reality of other people all you like. A more open-minded scientific approach is to listen to other people's experiences. People have some weird skills. And they believe some weird things. But yeah, it is hard to truly judge the skills of others.