To see the snark, examine the logic
1. After the filibuster, only the extremists will win.
2. Most of the winners will be Republicans.
3. [Unstated.] Most of the extremists are Republicans.
What counts as "extremism"? In this context, it has to do with how we think about judges. (And executive nominees, but I'll leave them to the side for simplicity's sake.) The "extreme" should be understood as the more ideologically slanted or threateningly powerful individuals that the President would otherwise have refrained from nominating. But even with the minority party disabled by the inability to filibuster, there are political constraints.
Obama can't just nominate, say, Bill Ayers.
He won't want the criticism, and there will be pressures on members of his own party to say no. The old game of letting the minority party do the dirty work has changed. The other party will still do what it can to trash the reputation of the nominee, but the President's own party will have to vote that nominee down or take the political heat for voting for this awful character.
I suspect that the political check will be more of a constraint on Democrats, because it seems that American voters perceive conservative judicial ideology as more conventional, proper, and neutral than liberal judicial ideology. And this is essentially the insight in the Slate article (which is written by Eric Posner). And by essentially, I mean subtract the subterfuge in the part I've boldfaced:
Next time Republicans control the presidency and the Senate, they will appoint ideologically extreme judges. True, Democrats could cancel out this effect by appointing extremely liberal judges when they are in power, but recent history suggests that Democrats do not care as much as the Republicans about appointing ideologically extreme judges. Unless this changes, picture a federal appellate bench composed of numerous Antonin Scalias and Clarence Thomases, not fully offset by Elena Kagans and Stephen Breyers.Let me restate the boldfaced part to say what I think is true: Democrats know that the vigorous left-liberals they'd like to see on the bench would be viewed by the American people as ideologically extreme and unsuited for judicial work.
The reason the Republicans seem to get away with leaning further toward conservatism than Democrats can lean toward liberalism is that conservatism better comports with the people's idea of the role of the judiciary.
Removal of the filibuster helps conservatives not because they are more "extremist" than Republicans, but because the political check on nominating strong judges operates more forcibly on liberals.