Apparently this is less true for newer reactors, or older reactors with upgrades. I'm not an expert, but https://www.powermag.com/flexible-operation-of-nuclear-power...

Even if reactors aren't as fast to react as a gas peaking plant is, perhaps batteries will soon be able to bridge that gap. The economics of batteries change greatly if you only need them to carry the load for tens of minutes for reactors to ramp up, vs needing to carry the load until the sun shines again.