The camera looks to the wrong direction (verticaly) when using the mouse. To solve it, in Camera.gd, I'm changing the _input_relative parameter when calling the update_rotation function to Vector2(_input_relative.x, -_input_relative.y).
It was intended at the time of recording I think. It depends on the default rotation that you want. Some older games used Y inverted by default (moving the mouse down looks up), and I don't remember which one I used in the tutorial, but it might be that. While now the convention is moving the mouse up looks up.
Thanks for sharing!