Klein is not "apologizing" for the earlier WaPo piece enumerating the lies in Ryan's speech. He's confessing that he was wrong for initially opposing it on the grounds that it appeared (to him) one-sidedly critical.

Don't condemn him for finally recognizing that journalists (himself included) have been conditioned to want to give both sides equal praise and blame -- and that sometimes one side are such blatant liars that you have to toss that conditioning out the window. More bloggers, columnists and reporters need to come to that realization.

He's saying, quite explicitly, that when dealing with the Romney-Ryan campaign, "looking" fair is not the fair thing to do.

Reread the entire column, DF. 

I think all of you are missing the main point of this essay, picking out stuff that refers to your own favorite peccadilloes about the press and not seeing what he's trying to say:

he's "confessing" that he just admitted to himself that Ryan, and the Romney campaign, are liars like he hasn''t experienced before in our politics:

After rereading Ryan’s speech, I went back to Sarah Palin’s 2008 convention address. Perhaps, I thought, this is how these speeches always are. But Palin’s criticisms, agree or disagree, held up. “This is a man who has authored two memoirs but not a single major law or reform — not even in the state Senate.” True. She accused Obama of wanting to “make government bigger” and of intending to “take more of your money.” That’s not how the Obama campaign would have explained its intentions, but the facts are the facts, and they did have plans to grow the size of government and raise more in tax revenues. Palin said that “terrorist states are seeking nuclear weapons without delay” and “he wants to meet them without preconditions,” which was true enough.

that

This has been a central challenge during this election.

that the Romney campaign is not just seeing things differently, not just spinning facts, but just making facts up to create an imaginary phantom to run against.

He even gives examples of the kind of advocacy argument they could make against the Obama presidency that he would be glad to give equal time:

For instance: Obama really has expanded the size and generosity of the food stamps program. He really has been picking winners and losers in the energy sector. He really does intend to raise taxes on the rich. He really does foresee the federal government spending more a decade from now than it was spending five years ago. He really did push an unelected board of health-care bureaucrats to make decisions about Medicare reimbursement rates. He really did want to raise the price of dirty energy. He really hasn’t released a plan that would ever balance the budget. He really did break his pledge not to raise taxes on people making less than $250,000 when he signed the Affordable Care Act.

He's telling his readers that the Romney campaign has changed the game. That he's used to it being political war rooms spinning narratives against each other using cherry-picked facts--which after all is an advocacy system, no different than in our courtrooms and in democracies everywhere --but Romney/Ryan are starting with the narrative and just making up the facts to fit it. That it's not even possible for journalists to play court stenographer with this team, because they aren't playing the courtroom game, they are playing a fairy tales or science fiction game.