This image you posted with 2.4k damage is not from 28 minutes into that game (which you haven't even linked in your post). There is no time, no max health, and no stacks shown in your image. I've seen the image for the game you're referring to at 28 minutes - Cho had 4.8k health without Stoneplate, had 15 stacks, 10 kills, and his ult did about 1.5k damage during Stoneplate's active (he got about 140 additional damage from AP).
Now, remember, this is 28 minutes in with 15 stacks and 10 kills. He would have gotten his ult around 6 or 7 minutes in, which means he has successfully ulted 15 times in 21 or 22 minutes. He was using his ult basically on cooldown and somehow got a kill with it each time (10 player kills + 5 minion/monster). That means that in the early game, before he could purchase Stoneplate and where he would not have gotten the full effect (it requires 3 nearby enemies to get the full power), he was destroying his opponents, anyways.
The game was pretty obviously skewed in his favor and not a very good sample for Cho's power. This was one game where Cho was dominating anyways and was able to get a massive lead even before his Stoneplate power spike. Most games, Cho will not be able to get that fed that quickly, and most games will end before he can get there naturally.