He is using a filter operation. If he was using the map then you would be correct, the ' would be replaced with nothing and his test would fail. But the filter only checks the condition, it does not modify the input.

where is your logic in this? have u ever played chess?
pawns only move foward
the only condition for them to move diagonally they have to capture an enemy Piece

or did the autor of the kata do mistakes?

Because ' or ''' is invalid.

I dont really get streams right now
so one question:
why u replace ' with nothing? wouldn't be won't to wont?
which in the test is not correct