You might want to re-read my post. I specifically pointed out that shooters are the standard example of where you can't do everything server side, where as traditional MMOGs you can.
Being able to directly set your position in an MMO is not the only way to cheat. Some MMOs have rules against plugins and helper tools. Those are also cheats. You can hack the client and display more in the minimap, or show you exactly how much HP some monster has, run a bot to control your character, etc.
Doing everything on the server only prevents god-mode types of cheats. You can also cheat with extra information, computer assistance, etc.