To be fair to him, we do know it's possible to convert carbon to diamonds in less than a year. In fact, I seem to recall a recent article saying that some people either are, or are very close, to fabricating gem-quality diamonds.
OTOH, I see a trainload of coal going to the local power plant every couple of days -- a process that is repeated for who knows how many coal-fired power plants. That translates to a LOT of trees. If you want to severely constrain the amount of time available for those trees to become a coal seam, a quick computation suggests that those trees would have had to grow in unsustainably dense forests. (To support the last 75 years of coal use, I got something approaching 15 billion trees, which would have had to be concentrated in the areas already mined for coal.)
Once again, I award you an A+ for effort. That did take some digging and research to make this argument.
Giggle, what is the Reynolds number for Carbon in a high temperature and extreme pressure chamber, before it is converted into Diamonds by man?