Following the link to the study leads to this summary (excerpt):
Bioenergy is an inefficient use of land to generate energy.
Fast-growing sugarcane on highly fertile land in the tropics converts only around 0.5 percent of solar radiation into sugar, and only around 0.2 percent ultimately into ethanol. For maize ethanol grown in Iowa, the figures are around 0.3 percent into biomass and 0.15 percent into ethanol. Such low conversion efficiencies explain why it takes a large amount of productive land to yield a small amount of bioenergy, and why bioenergy can so greatly increase global competition for land.
It seems the study did not even consider any new approaches to making biofuels.
http://en.wikipedia.org/wiki/Second-generation_biofuels promise the ability to use material that would otherwise be waste, such as straw, thus lessening the competition between food and fuel. Any study that claims to make forecasts for the year 2050 (also in TFA) should take a serious look at these too.
The study in TFA only gives a cursory overview over second generation biofuels with an either-crop-or-cellulose point of view. It almost seems that the option of using crop residue was intentionally neglected...