That was true in the 70s. I think anything built in europe since the 80s can do load following and France does that every day. I think by regulation new plants must be able to adapt their production by something like 5% per 10min (or something of that order of magnitude), well within the range you need to meet intra day variations.

Now whether it is optimal economically is another question. If you have some sources of energy in your grid that cost per usage (eg fossil fuels), you should rather switch those off than nuclear which costs the same whether you use it or not. But if your grid is almost all nuclear (eg France), you do load following.

> but it doesn’t scale up or down quickly.

The newer ones can. Though that's partly my dislike. There's always yet another new technological solution for the various drawbacks. Which often are unproven (e.g. SMRs) and that often results in crazy cost overruns.

Nuclear has a crazy high fixed cost so ideally you still run it continuously.