That's obviously advisable when testing in-situ wiring in a situation like this. However, he has tested just a length of unused cable on a drum, with no accessories or anything else connected - and still got a 'low' IR reading (although he gets the expected >999 MΩ when he tests another in-situ circuit which uses different cable, and when he leaves the meter probes not connected to anything)I would also suggest removing the accessories during your installed cable tests, just to confirm it is the cables with low IR.
Kind Regards, John