This was 5-6 odd years ago and he no longer works there, so things might have changed, but based on this tweet it seems unlikely.
This seems to be a common theme with problems at Valve.
Our lead says that all work must come from the scrum master but in practice it is selectively enforced.
The typical problem at software companies is that developers are incentivized only to write code for new features that will land them promotions and look good on their resume--but bugfixes and security work is not part of that.
Management can counteract this with top-down initiatives. Programs like "fix-it week" or teams dedicated to security with different incentives in place. For example, Google suffers from the "promotion-oriented programming" about as badly as any other company, but they manage to take security seriously.
Valve has "flat hierarchy", which goes in quotes because the hierarchy isn't really flat, it's just hidden. Because the hierarchy is hidden, it's harder to address large-scale problems like institutional priorities... because there are fewer people to delegate large-scale problems to.
There is even reason for (say, for example) Rockstar to leave hackers alone in GTA : they act as artificial whales to lure real players into buying in-game currency in order to keep up/seek revenge.
There are a few games I can think of off the top of my head that have a symbiotic relationship with hackers.
The only way to prevent this is to remove elevated access from the player's computer. This has been done with varying levels of success on consoles, but even then it's only a matter of time.
But I also think a lot of the hackers in both GTA and CS are cheating in ways that no regular user input could trigger, they're compromising the software at a lower level than that.
The bigger problem is that even with input recognition, one of the biggest problems are wallhacks, meaning you can see other players through walls which is an advantage that's almost as large as aimbotting in tactical shooters like CS.