Yeah, a lot of people don't know in detail how gerrymandering works. For it to work you need to pack as many D voters as possible into a small number of districts and add a few extra R voters to all the remaining districts. You have to dismantle the safe R districts because you need to distribute those voters among the other districts.
In a standard election that works because the Ds get a huge win in a small number of districts and the Rs get a bunch of tight wins in a larger number of districts, so the Rs win.
But, when there's a huge wave election, the small number of R voters they added to districts aren't enough and not only do the Ds get a huge win in a few small districts, they also win some of gerrymandered R districts. There's nothing safe anymore.
If you don't gerrymander, you naturally get some safe R districts, some safe D districts and some toss-ups. But, gerrymandering has to eliminate the safe R districts to shift those voters around.