That the mere existence of rules is not enough. They must also be enforced.
Basically, I think you're letting Valve off the hook. Sure, the competitors shouldn't cheat, but Valve should also make sure they create an environment in which cheating isn't incentivised. In the world of software, a few years is a long time. Leaving a known bug that gives a massive competitive advantage unfixed for that long borders on negligence, especially when the fix is relatively trivial.
They are enforced and, again, this exploit was an example of that.
Yes Valve could do a better job at supporting esports. But there will always be in-game exploits as well as out of game methods to cheat so ultimately it is down to the individual participants to cheat or not.
Also these tournaments are run by third parties on third party servers. Valve isn't even involved except for a small role in the once a year majors (that haven't happened for like two years now). So saying this isn't enforced just because Valve didn't fix a bug makes no sense because Valve isn't the company that enforces these things anyways.
Basically, I think you're letting Valve off the hook. Sure, the competitors shouldn't cheat, but Valve should also make sure they create an environment in which cheating isn't incentivised. In the world of software, a few years is a long time. Leaving a known bug that gives a massive competitive advantage unfixed for that long borders on negligence, especially when the fix is relatively trivial.