I think it has more to do with the 15% downturn in German industrial output over the same timespan due to the reliance on Russian gas now having to be replaced with LNG.
Wrong. 2.02 billion m³ of the total 2.56 billion m³ reduction came from just nuclear powerplants. The rest of the energy sector still uses 6.59 billion m³ but considering there were barely any reactors still running in 2019 this is a massive reduction from just turning off those few reactors.