> the author is looking at nameplate capacity, which is a completely useless metric for variable electricity production sources

For solar panels, the nameplate capacity is usually also the power generated at the peak production time, which is the moment when an attacker turning off all inverters at the same time would have the most impact.

That is: for an attack (or any other failure), the most important metric is not the total power produced, but the instantaneous power production, which is the amount which has to be absorbed by the "spinning reserve" of other power plants when one power plant suddenly goes offline.