It is

And is correct that way because inputs "... are always lowercase strings, and that each has at least two letters.". If that was not the case and arbitrary inputs would be allowed we'd also need checks for null and lengths in order to avoid NullPointerException and IndexOutOfBoundsException or alike.

it work only if cases are equal

it is not 4 kyu

This is perfect ;(

return a.Any(b.Contains);

if array is null it doesn't work

overload )) I thought the fork was removed ))

So, you fork a Kumite titled "...without converting to string", and put a string conversion in the code..

Perhaps fork one of the ones that have done this predictable, inefficient approach to death..