You moved the goalposts from a piece a day to a bell curve of volume-- and added metal poisioning. No, a piece of chocolate a day would alone cause neither obesity nor diabetes.
If all people took that non-fact to heart and started eating a piece of candy a day that they weren't eating before, without changing the rest of their diet, then we would have more people with diabetes and other health conditions, all without moving any goalposts.
That rough calculation won't be highly accurate for a number of reasons, but you could halve it and it still is very bad, thus it goes to show how easily fat can accumulate once maintenance calories are breached. Take care of the pennies and the pounds add up. Same goes for calories.