Whats your node hierarchy like? You could always use a intermediary node2D as parent of the turret to do the correctional rotation there, then control the turrets rotation from code. The intermediary node would always inherit parents rotation and the turret in turn the intermediaries... if that makes sense.
edit: also note that kenny.nl assets are built specifically for unity, so that might introduce the "issue". I don't really recall, but unity might have some of the axis flipped so that what is positive direction would perhaps be the negative along that axis in godot instead..