Love Steve Magnus and his deep dives!
In the case of “non-responders,” I tend to think of individuals who don’t respond to any training stimuli. In 25 years of coaching I can think of only one athlete, that no matter the stimulus, they just never improved (ok, they technically improved but talking like a second or two over 4 years for a beginner time in the 800). More volume… nope. More intervals, tempo and threshold work… nope. Anaerobic sprints, weight room, hills… nothing worked. Maybe it was they were already at their genetic ceiling? Some will respond better to intensity, some volume. And occasionally nothing at all.