Including Chernobyl and Fukushima?

Sure. It is an effect that has been taken into account by government agencies and grid operators, though: https://www.eia.gov/todayinenergy/detail.php?id=61743

The good thing is you can predict it extremely accurately, probably much better than cloud cover…

I have no idea about the actual number, but saying the power is decreased by 30 GW does make sense, though… Of course it is not energy, but they might not have meant “solar energy” in the sense of a physical quantity. The sentiment is bullshit of course.