It depends on how you define the modulo operator [0]--there are definitions where (-1)%5 == 4 and definitions where (-1)%5 == -1. Presumably the GP is referring to the former case.
I don't follow your point. We're concerned with the value of -1 % 5, not 1 % 5. There's no definition of the modulus operator on which -1 % 5 = 1.
Phrasing issue, I think. I understood "Negative `i` indexes the element" as `i` itself being positive. Like you'd say "negative 1 indexes" (i is 1), not "negative -1 indexes" (i is -1)