Trees turn into peat which lithifies into coal. The anoxic and acidic conditions of peat bogs can slow decomposition enough to allow peat and coal to form so not all coal is from the Carboniferous, just most of it.

Also the evolutionary lag hypothesis that attributes that extra Carboniferous coal to a lack of lignin decomposers has been thrown into doubt.

Ah cool. But it’s not dinosaurs that’s for sure.