I think failure is key for muscle growth. Why would your body response growing stronger, if you didn't push it to where it NEEDS to become stronger. However you can still hit failure at higher reps. I prefer to not go under 8 reps, to prevent injuries associated with heavy weights..
George Farah trains some of the best. Yes. But also at the same time the most genetic gifted ones. I really don't think the type of training Kai does would have an extreme effect on his body. The guy is growing no matter what he does.
If I'm not mistaken, Kai kind of plateaued on his legs a few years back until he started going to high reps, like 20-30 per set. Definitely supports that going super heavy with low reps is not always the solution.