What kind of capacity can Germany rely on to receive from other countries when it's own production is zero? Someone has to have reliable capacity somewhere, usually basic infrastructure such as electricity is built on guarantees, unless Germany has decided it's okay to get blackouts when it's not windy and neighboring countries would rather use their power themselves
There are no guarantees in life, only probabilities. People much smarter than us can determine how much spare capacity the EU needs from non-intermittent sources to make electric blackouts a 500-year event, just like they do with building standards related to earthquakes.
I don't know what the number is, but it's probably greater than 0 and definitely less than the total peak EU demand, and if I had to guess I would say it's probably closer to 20%.