> Despite feedbacks with weathering rates, much smaller imbalances would have resulted in the complete removal of atmospheric CO2 in less than a million years. Without evidence of such dire consequences, lignin production in the absence of lignin decay for more than 100 million years into the early Permian is untenable.

The study addresses CO2 levels closely, as well as other significant pieces of evidence. I would tend to assume the peer reviewers at one of the world’s most prestigious journals thought to Google up historical CO2 levels.