It happens _all the damn time_ in winter and late fall. Anyone who doubts that is conjuring their own reality.
Poland had 2x larger share of renewables in July than in November.
There's barely any sun, wind increase isn't that significant to cover lack of it, and energy demand is very very high.
A reduction of only 2x between July and November is actually super positive for renewables. Wind and solar energy is almost free on an ongoing basis and installation of new capacity isn't expensive. Your number suggests that simply installing 2x the capacity needed in favorable weather would be sufficient for unfavorable weather. Actually it's more like 3x but that's still often very doable.
The researchers used past weather data (as you wrote), and looked at how that would work in a future (as I said) energy-mix scenario: a climate-neutral energy mix as projected for 2045.
So it definitely does not refer to the energy mix as currently installed.
Where did I claim anything about the currently installed energy mix?
I claimed that no wind and no sun happen very seldomly across Europe.
The aspect of the 2045 scenario is irrelevant here.
Btw. it's important to note that there are more renewable sources besides wind and sun: heat from the earth, water (e.g. rivers, tides), bio gas, ...
Which fortunately doesn't happen too often in Europe.