I stand corrected

It does.

When both letters are in upper case, both calls to char.IsLower() will return false.
false == false resolves to true, so the function returns 1.

This does not account for the case when both letters are upper and should return 1.

For some reason, the random tests in the C# translation almost never fail, and you can return pretty much anything. I was able to submit a solution that has hard-coded return values for the three visible test cases, and just returns 0 for all other tests:

https://www.codewars.com/kata/reviews/5e5904021cff490001cdb237/groups/64ca1a6ca9a9ec0001fc2fb8