It seems clear to me that the BIRs wouldn't marinate for 3 or 4 days, but I can see why Jerry takes this route as it does allow the marinade to penetrate better and must help with tenderisation of the meat.
I suspect that the BIRs with the tenderest tikkas are using chicken that is pumped full of water as I would imagine that that would both soften the raw meat and allow the marinade to penetrate better in a shorter time.
I can confirm that the Iceland 3-bags-for-a-tenner chicken breasts come out very tender, and guess what, yes, they're pumped with water. And BTW, you chicken snobs, try these Iceland chicken breasts, I think you'll be pleasantly surprised.