Misinformation. Fungi were around big time in the Carboniferous period and had been for hundreds of millions of years. There is plenty of evidence that decay of lignin by fungi and bacteria was routine in the Carboniferous. Coal measures were formed via the very different environmental conditions pertaining at the time. A hot, wet climate with high carbon dioxide levels favoured the rapid growth of forests. Initial burial of fallen trees in acidic swamps, resulting in anaerobic conditions unsuited to decay organisms, were probably the major driving force, with tectonic burial of future coal deposits also playing its part. Certainly there was a peak time for coal formation, but that wasn't the end of it. There are tertiary coal deposits a few miles from me that were once exploited on a small scale.