1 point*woosh* You missed the joke! Also that it's subtly making the point that it sounds absurd to just say "concepts aren't in the brain" as if a simple concept like "guys" wouldn't even be real because it's not "in the brain" and therefore arbitrary.
1 point(putting on grammar nerd hat) As relevant here: "Guys", plural, is synonymous with "people" and has no gender implication, as contrasted with "guy", singular, which is synonymous with "male person". So his "Hi guys!" was no more than a friendly greeting to whoever might read his post.