Valve's anticheat is atrocious, Counter-Strike is an absolute cesspool (so much that they've had to separate players who bought the game from players who play the free version), and Team Fortress 2 has had more bots than players for like 10 years now.
This seems like the kind of thing that is repeated so much because some people want to believe it, and most of the time I bet it's people who haven't played a Valve multiplayer game ever.