Not taking sides in this debate (which is over whether or not nuclear power should be used), however the paper has four authors and in the introduction they say:

"The main purpose of this study is to explore whether low-cost mixes using 100% WWS can match energy demand with intermittent supply, not to prescribe the only possible future energy mix in each region. Indeed, many different future scenarios can be constructed depending on the objective. The objective here is to determine whether a 100% WWS system, which we believe maximizes air quality and climate change benefits while minimizing catastrophic risk relative to systems using other technologies, can deliver energy reliably at a reasonable business cost worldwide."

So either way, if their modeling is good, it is still a useful assessment of options.