Viren ran 3x/day, upto 180 mpw at 7000 feet in Kenya. He was way ahead of his time training at altitude in spartan conditions 40+ years before the Robertson brothers. So yeah, he did have extra RBCs, from altitude. He did this for both Olympics. The other half of the year he trained in extreme cold on rough root laden trails, giving him a light quick stride used to changes in pace/conditions.
He was physically a freak, had a 38 inch inseam and yet his legs weren't bone thin, they still were quite muscular. Small upper body with stick arms. Bad for MMA, great for distance running. I've lost the link, but recall his easy runs were approx 7 min/mile, but HR was 95-100, so very easy recovery pace, yet well beyond hobby jogger pace!
Old fashioned blood doping only lasts so long, you have to dope shortly before your main event. You can't dope all year round, the body detects the extra RBCs and shuts down it's own RBC production. Yet Viren was setting PBs and national records long before Munich Olympics. This does not prove, but seems to indicate, he had the talent before the olympics. Yes, he had off years in between olympics, due to injury.
Lastly, he was an exceptionally astute running tactician and always knew how to run the minimum distance, *and* force his competitors into running extra distance, as did Quax and Dixon in the oly races.
Quax et al were just prideful and jealous, and couldn't admit to themselves that they didn't work hard enough or didn't have the talent to beat Viren. Kinda sounds like a lot of runners today, pointing their fingers at runners coming out no where to do well. Some accusations are correct, some are not.