They are saying that if 50% of your generating capacity drops to 1% of you need a lot of gas powered plants to make up that difference. If your renewables represent 1% of the generating capacity you need very little gas to make up that difference.
100% Gas will use 100TWh of gas to generate power
50% renewable will use 50TWh of gas to generate power
You go to 60% solar 40% gas.
Obviously the numbers are made up but that’s the point I’m trying to make.
If your example you've reduced generation from fossil fuels from 70% to 40% so still a win.
I assume you're still going on about nuclear. The problem with nuclear is that it doesn't make financial sense even if it can run at 100% of capacity for 100% of the time, it certainly can't cope with variable load - you can't scale nuclear to provide your peak amount, so you need to be able to top up with gas or battery, just like renewables need top up