then I would agree

you are right, my comment was more about the outside world

Then the expected result would become dependant on the system. A kata should try to avoid that in my opinion.

better to use Environment.NewLine for cross-platform support

I disagree!
The task is to fix the code, not to rewrite it.

How it works?

This is so slow. Simple foreach loop is more than 10 times faster.
You have to do 2 iterations here. With loop you iterate only once.

Fun kata, but you should make it easier to know which pieces of code are Preloaded by copy/pasting them into the Description.

I like the check of .Min() == .Max() -- that's clever.

C# version has random string length tests, eventhough the description says otherwise.

"Both strings will be of length 18, and each character in the string will be a number between 1 and 9 inclusive."