It's hard to say, because he does say that even on food, the vitamin dies quickly after the fruit becomes detached from the plant. But his main peeve seems to be the use of synthetic and isolated vitamins, as opposed to what is in the fresh fruit itself.
I understand also the the acidity from citrus is somehow alkalizing to the body. I'm not sure how this works, or if it applies to all citrus or limited to lemon. I'm jusy wild guessing here as to what he may have meant by this seeming discrepancy, but I would be very pleased if he corrected me himself.