We're they keeping the dirty generation at surplus level for safety precautions, due to renewable sources unreliability? If yes, I think the math is misleading

We're they keeping the dirty generation at surplus level for safety precautions, due to renewable sources unreliability? If yes, I think the math is misleading. It doesn't matter if you generate 97% from renewables if you can't trust it and still need to keep dirty sources, even during renewable peak...