Tissue and Lewis continue saying that "glacial plants were severely carbon limited over a very long time period, until atmospheric CO2 began rising during the glacial-interglacial transition." In fact, they indicate that "controlled environment studies with modern plants grown in glacial CO2" have shown "significant carbon limitations on plant physiology even when other resources were generally not limiting ," citing Dippery et al. (1995) and Tissue et al. (1995). So in spite of anything one could have done to enhance their productivity (other than supply them with more CO2), glacial-age plants simply could not produce the bounty that today's plants do. In fact, they were fortunate to merely survive.
Fallacy: oversimplification