It's impossible to know the counterfactual, because having a set date for leaving nuclear has also been a forcing function for ramping up renewables. So it's entirely possible that coal would have made up as much or more of the energy consumed in the hypothetical 2021 with nuclear plants still active.

Of course there could have been an even more radical counterfactual where Germany decided to get out of coal by doubling down on nuclear but that hasn't been a political possibility in Germany for many years.