Believe it or not, there exist some programmers who have never taken a course in discrete math. ;)
Not really. The argument isn't that "x not in y" isn't useful, but that it duplicates "not (x in y)" by adding an operator spelled "not in".
I like "not in" and use it lots but I wondered the same thing the first time I saw it.
That might be the argument elsewhere in the email thread, but it's not apparent from the actual page linked to. Without any other explanation, I'd go with the assumption that the poster thought that this particular email contained an interesting idea, and submitted it.
It was not covered in my discrete math course.
We didn't have a "CS discrete math" course. We took the same duster te math and algebraic structures course as math majors, then we had a languages/theory of computation course that covered the CS-specific bits.