Suppose it depends on who has the files and what their knowledge of coding is and what their intentions may be. Valve aren't completely ruling out potential problems, they are saying they can't guarantee issues won't appear and it's best to play on 'official servers' basically covering their asses.
A lot of the vulnerabilities in the build will have been patched already, but I guess this doesn't leave new ones being exposed.
Because of the profits involved, you also can't entirely rule out some form of damage control in the media by Valve.
According to some accounts the rumours have hit the player base of CS GO quite hard.