The implication here is that the Democrats don't gerrymander. This is patently false. The Democrats are just pissed that the Republicans did a massively better job at it in the past decade.
In many ways, it was an arms race, and the Democrats chose not to focus on small and local districts and elections (instead, worrying about the "big picture"). As it turns out, politics trickles up, and small wins for Republicans eventually turned into big wins. To see a perfect exemplar of this, just look at Jon Ossoff's crushing loss last week.
There's an excellent NPR podcast about this very issue I listened to a few years ago.