Once, ages ago, I tried to correct the math of the Editors of Shacknews' math and like you, I was wrong. I made it to the front page of ShackNews the next night, haha. I used the same user name as I used on a forum with my friends, so my friends all found it out, haha.
CO2 is 72.7% oxygen by weight, and 27.3% carbon. 2kg is 28.5% of 7kg, so with rounding it works out even if that is assuming that the coal has no impurities - it may be closer to 6kg but essentially it takes oxygen from the air and adds it to the carbon in the coal to form CO2
Politics last two decades acting all surprised when scientist and ecologist been telling them for more than a century... Silver lining is they won't be lying for another century if they keep ignoring the issue, we'd all be dead