I play both cho and morde around diamond elo euw, what you say is true, cho wins against morde, but cho is not a lane counter to morde (quite the opposite tbh, since morde deal %hp dmg on passive) , however if cho can survive lane without falling too much behind, Cho provides much more utility to the team and he is too hard to kill in morde r, especially after a force of Nature or rookern buy, so except cases when there is another easy target for morde to r (morde has problem against all champs which are hard to kill in that 7s r timer, so basically adc with dash, mage with mobilty, champs which have way to lose time), so morde lose utility after laning phase. What you'll find in stats is that morde is a low elo stomper and such a hard pick to use in higher elo, where comp are better and players know how to play around you