There are so many factors that contribute to improvement that simply saying "we both were hard workers so why didn't he improve as much as I did" doesn't do justice to all that goes into improvement. Sleep, nutrition, external stressors, balancing maximum efforts with active recovery days, etc all play a role in improvement. I haven't read the book but plan to but it seems like unless everything was the same in the majority of those categories, its hard to make the claim that some people are "untrainable". I think we all know that some people are more genetically gifted than others but especially when it comes to running, there is such a science to training programs that maybe his teammate was overtraining and that's why he wasn't improving as much as he was. Or maybe he was getting an extra hour more of sleep on average and that was improving his recovery, etc. I'm just not buying that "the data says training is overrated"