"The fact is that all coal reserves were laid down before fungii evolved."

Not so. There were plenty of fungi around in the Carboniferous. It was also likely that fungi were digesting lignin (the stuff that makes wood wood). What you've probably read is that fungi hadn't evolved the ability to digest lignin. But that can't be right. Read this article:

"Delayed fungal evolution did not cause the Paleozoic peak in coal production" (it's at pnas.org).

In a nutshell, had lignin (wood) accumulated at the rate it would have had to in order to produce the coal measures (which it did), and not been broken down by fungi (read on), all the atmospheric carbon dioxide would have been removed in a short time. Lignin is a carbon-rich polymer, after all, and you're talking about wholesale carbon capture on a massive scale. The numbers are in the article. So no carbon cycle, no photosynthesis, no life on earth and a drastically reduced greenhouse effect. Didn't happen. Some of that trapped carbon must have been released back into the atmosphere, and that happens by the action of decomposer fungi and bacteria. The latter, by the way, are even more problematic when it comes to digesting lignin. The likeliest explanation for the peak in coal formation is a unique combination of climatic and tectonic events. And maybe a bit of a contribution from fungi scratching their heads wondering how to deal with this new-fangled polymer. They got there, but they probably got there before the explosion of woody plants happened.