Dunkelflauten (and seasonal leveling) are covered by use of an e-fuel like hydrogen.
You can see this effect in action at the optimization/simulation site https://model.energy/
Go there, solve for Germany (2011 weather data, 2030 cost assumptions), then disable hydrogen and try again. The optimum cost nearly doubles.
Interestingly, this simulation also suggests a 95% cost increase if Germany tried to do it without e-fuel/power-to-X (which it estimates would need to cover about 8% of demand)
Battery storage typically seems to smooth out production over about 24hrs, not multiple days and certainly not seasons.