It's not wrong. If you read the intro, they note that those traits are implementation defined, so they say to assume you're using an implementation with those characteristics.
So this is incorrectly titled. This should be a "A Quiz About Integers in C on common x86 and x64 ABI."
Unfortunately this does not educate people to that kind of issues, which is very unfortunate.
The assumptions are explicitly stated at the beginning of the quiz.