I think it means - just as closing your eyes doesn't mean the room becomes empty, wiring the learning network randomly doesn't mean you'll end up with no pre-conceptions (e.g. the rule system at least will need to be programmed in).
I think he's suggesting that any given random set of preconceptions will itself be a 'sample' from the entire preconception-space, and therefor itself a preconception - just one that you didn't involve yourself in choosing.