"Mean" has a very particular meaning. At least it seems to in this particular essay, if I may speak on behalf of PG regarding his use of it. He even hints at what it may be without explaining it directly:
Children (especially children of PG's age - both under 7 or so, I believe), when fighting with or teasing or blaming each other, are not usually described as "dicks" or "evil" or "unethical", or even "rude". They are too young to have those properties ascribed to them. It is common, however, for children to call each other "mean" while fighting, and it is also common for parents to tell their children "don't be mean" (as opposed to "don't be a dick", which is what teenagers tell their friends, or "don't do illegal things", which is what courts tell everybody). So when PG uses "mean", he uses it to describe behavior in adult-ish YC founders that is isomorphic to little kids being bad.