Nuclear can deliver flexible demand if required, it just involves either lowering the reactivity in the core, or if the drop in demand is sudden, bypassing steam from the turbine and running it directly to the condenser. But since their operating costs are so much lower than their construction costs operators run them at 100% capacity as long as they can.

The only situation where deliberately operating a nuclear plant at under 100% output is when nuclear makes up a very large chunk of a country's generation capacity. It's not that only French nuclear plants can reduce output it's that only the French have ever been in the situation where their nuclear capacity exceeds their minimum electricity demand.