Shen True Damage

I was in a customs game with a friend to test the new Shen, and upon killing him it claimed he had taken over 1k in true damage.

He was Cho'Gath, and it was about 46 minutes into the game.

I just grabbed red buff and then we fought. Upon his death he reported my Basic Attacks dealt true damage. Is this to show percent health damage? Because if the true damage was dealt via red buff it would normally say so would it not?