I'm a long distant runner turn crossfit and could never understand this high/low impact stuff. and I do agree with you, what this trainer proposes seems like an avoidance of the problem and not a remedy to it. As for the impact of running, I never considered it an issue. Up to this point, I always thought high/low impact only applied to Aerobic sessions.
In my, not a trainer, opinion, everyone, with the physical ability, can run...we did it as children we can do it now. a direct component to the good ol' fight or flight response to danger. Most running books I have read and running coaches I have talked to, stress form to make running efficient, not so much as to make it possible. Everyone, if they run long enough will generate a more proficient form through time, at least one that is proficient to them. I think the use of a tredmill or other cardio device does nothing for form or adaption to the impact stress associated with running.
I used to have shin problems but a thousand or so squats later and i am good. man, i love crossfit, If I would have done this in High school, I probably could have broken sub 14 minute in cross country and sub 9 minute in the 2 mile.