You might not be able to turn nuclear on/off very fast but adjusting from ~5% to 100% and back is really fast. Actually faster then most gas peaker plants in W/min change rate.
This is not really done much outside of France as the running cost of a nuclear plants are roughly the same at 5% and 100% of output (actual fuel costs are very small part of the total cost of running a nuclear power plant)
These paper companies own all this production because electricity is a massive part of their operational costs and by owning the production if the market goes bad they can just use it themselves at cost of production instead of selling the electricity on the open market basically capping the cost of electricity in their costs.
(edit: Actually now that I think about it this is very similar to how most big cloud platforms work. Basically you build infrastructure for your own use and then also sell the same infrastructure to anyone who is interested.)
> Producing 5% of 1MW can be costing you more than what you made selling 100% of that 1MW for most of the day.
The thing is that with nuclear the costs will be mostly the same when plant is off, at 5% or at 100% (probably most expensive is at off state as you can't use the electricity you made yourself to run all the cooling systems etc). Obviously if the market prices are negative you just run at plant at minimum power which is basically just enough to cover the plants own usage.
This happened because we removed base load generation from coal and nuclear and replaced it with gas. The only thing which can turn on fast enough to match clouds movements. Then went and bought that gas from geopolitical adversaries.