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I think what @sheogorath means is that coal or gas plant can respond pretty quickly (minutes) to a falling or rising output from wind or solar, while nuclear can change its output much slower (hours).
This is obviously a "problem" created by the very intermittent nature of wind and solar, but needs to be dealt with on the grid level.
In any case, CO2 emissions are much larger and long-term danger than benefits from fast power switching.