Cho'Gath ult should be considered a true execute

So Cho'Gath's ult does true damage (so it goes through damage reduction) and builds off of hp and ap (50% ap, idk how much hp). This means, like pyke's ultimate, it kills people no matter what if they're within a certain health range (the dmg of the ult). So why is it that pyke's ult is considered a true execute and will tell you on their health bar when you can just ez auto kill them, but it doesn't tell you with cho'gaths?

Instead, they've decided to surround the champion with purple teeth, like when he does his knockup. Now like most effects, there's a problem with seeing the teeth during big team fights so why not do what they do with pyke's ult and tell you on their hp bar when you can just instakill?