Some interesting back of the envelope calculations. One thing it does not, because it can not, take into account is innovations. You can get more power out of smaller areas because of increased efficiencies. More efficient solar powers, higher energy bio-diesel (selectively bread or bio-engineered) plants, and their equivalents across the board will shrink the footprint per unit energy.
What these calculations may prove/show is that we are not at a stage technologically where a full conversion to renewables is feasible or advisable. Fortunately, we are not at a point where a radical overhaul is required.
Also not mentioned but of potentially massive import, Thermal Depolymerization.
Video embedded below.