This is absolutely not true.

Right now Denmark produces 1724 MW from solar panels even though it is an overcast and rainy day: https://energinet.dk/energisystemet-lige-nu/

Our current usage is relatively high: 5944 MW.

Remaining supply is: 3458 from windmills, 357 from fossils and 434 from import from other countries (mostly hydro/wind).

So if Denmark doubled the amount of windmills (it is a very windy country) and solar panels then Denmark would be able to run of windmills and solar panels even in the autumn.