I have my suspicions about rampant doping but I'll throw out an anecdote and put it into some perspective.
I lived with a Kenyan guy in the late 90s who ended up running 2:08 high at a major marathon that is one of the fast courses where a record is possible. At the time the world record was 2:06:50, so he was legit, but he was never a big name. He had very modest track PRs (I don't know exactly but lets say 13:50 and 28:45, maybe not even under 14:00 for 5000m). I'm sure with track focused work, he could have gone quite a bit faster but all he really did was a lot of mileage. I think most would say his training wasn't good (obviously it led to good results but was missing a lot of the key elements of things people do today).
1000% this guy was clean, and would never dope. It wouldn't enter his mind. I believe that with todays shoes, and with a professional coach and todays training methods, he would almost certainly be a 2:04:00 or 2:05:00 guy.
Can I believe that someone can run 3:00 faster than that and still be clean? Maybe, maybe not. But I can believe 2:03 - 2:04 clean.
The real question we should be asking is why Americans are still struggling to break 2:10 when the rest of the world is 5:00 faster. If it was only the Kenyans and Ethiopians (mostly it is) I'd think drugs could be a big factor. But the Japanese are killing it too. Did the Japanese suddenly start becoming a drug suspicious nation or did something change in marathoning?
American track times are WAY better than in the late 90s but we still have weak marathon times and this should really be explored more. I actually think Mantz's debut time was poor and I'm surprised he positive split. The guy has run 27:25 and 13:10. I expect he'll improve a lot but...