There is a chemical difference, and I would assume it is this difference which causes its beneficial effects for me.
In terms of chemical reactions, the difference is extremely small. The chemical reactions will all be the same, but there might be a very slight difference in reaction rate. I think it's extremely unlikely that the deuterium instead of hydrogen is responsible for anything about Cellfood.
No, you can say that the safety of what @Mary's taking is proven. To extend it to all sulfuric acid health products is a bit too broad for me.
Cellfood is basically sulfuric acid with contaminants. That's probably what most chemistry labs would report. I expect it would be hard to notice that the sulfuric acid molecules are slightly heavier.