No, he's running as a Democrat. He's Hillary's only declared competitor right now (I think?).
In that case, the double standard lies in the Atlantic's reporting of O'Malley. These are Maryland's "proportionate" districts: Granted, and this is conjecture, I imagine that the politicians in Maryland were under political pressure to gerrymander because the Republicans do it so much. They control more state legislatures than Democrats, so Democrats in Maryland "did what they had to" to wrest one more House seat for themselves. It's deplorable but the reality of pressures and forces inherent to our government.
You are correct. It became one of the worst cases in the country because of it. At least one of the districts was done for no reason by a democrat, but the rest were more or less "playing the game." All of our borders are wonky as hell. It's been addressed a few times to being fixed, and probably will in the next few years. We will see.