"France's nuclear operators have been claiming this for years. But recently started claiming that wind and solar are bad because they force nuclear to flex which is too expensive." - one doesnt disprove the other.

French nuclear is extremely flexible https://www.services-rte.com/en/view-data-published-by-rte/g... but it doesn't mean it's free. Solar and wind without proper bess to support them are creating problems for other generators, acting as grid parasites without offering proper firm generation