Not sure I understand this one. I'm finding it difficult to read this as anything other than "yes, most people understand negation as negation, and not as something entirely different". Are there any languages or cultures where negation is same as inversion?

How would you even invert an adjective that doesn't exist on a one-dimensional scale? For example, good<->bad makes sense, because they are clear opposites. But happy<->sad does not make sense, because emotions don't exist on a single axis and do not have clear opposites. "Not happy" encompasses all states besides happiness. Could be angry, could be sad, could just be neutral. Like the old saying goes, "the opposite of love is not hate; it's indifference".

I'm NOT surprised. As a programmer I really miss having the antonyms that Ruby gives you in other languages

Having to parse what '!includes' means is harder than just 'excludes' and, while that cognitive load is small, it builds up!