I read the article and I do not see the 80% mentioned. I really would have loved to read this, but they mentioned only 10GW of variation, that's roughly 20% I believe.
Another article talks specifically about the power modulation ability: https://lenergeek.com/2017/09/13/modulation-parc-nucleaire-e...
That's actually not been true for the last 40 years or so. In France, nuclear power plants have been doing load following without problem. They are apparently able to adjust their production by 80% over 30 minutes. One such episode is explained here: https://lenergeek.com/2019/03/07/mix-electrique-nucleaire-tr...