Sorry but this is kinda silly. I mean, the overarching discussion of the value of upper body strength training for middle distance performance is a totally fair thing to have. But using kids like Kessler as any kind of barometer for what general athletes should do just doesn't make sense. Kessler is a generational talent. That's why he runs 3:34. Is it possible that the upper strength he's built from climbing has raised his performance from 3:36, 3:37, to 3:34? Yes, maybe. But he would be unfathomably fast on almost any kind of training.
Suppose Kessler had no background in climbing, had never touched a weight in his life, and ran 3:38 the other day. I could then point to him and say "have most of us been overestimating the importance of lifting in middle distance performance?" and have a seemingly valid point - who is going to argue against a high schooler running 3:38?
For the most part, there just isn't any sensible way to point to people who run world class times within a year or two of starting the sport and conclude anything about how we should train. All you can conclude is that these people are immensely talented beyond anything the typical athlete or coach will ever have occasion to deal with.