We don't technically know for sure that the engines on Titanic were reversed, as reports are conflicting. But what we do know is that if they were, it would have had a impact on how effective the turn would have been.

For a rudder to work properly, you need the water to be flowing past it in one direction, which would be moving aft along the ship. If first officer Murdoch put Titanic's engines into reverse, the water still being forced by the rudder as the ship moved forward would have been churned around wildly by the propellers trying to work against it. Without a clean sweep of water flowing around the rudder, it takes away from the effectiveness of the turn.