I'm not suggesting we assign 5,000km^2 to each individual nuclear plant, just in case they explode. I'm suggesting the 5,000km^2 that currently exists as barren nuclear wasteland should be included in the total land attributed to nuclear energy. That would bring down the average generated energy per km^2 pretty significantly.
The dataset does what I've suggested for hydroelectric. The dams are relatively tiny (and in fact, would outperform both wind and nuclear in this dataset), but they render huge areas unusable. The unusable collateral land is attributed to hydroelectric power, as it should be.
It stands to reason that collateral damage caused by nuclear should be included, if collateral damage by hydroelectric is too.