Given Tanden's background working with welfare reform champion Hillary Clinton, this should surprise approximately no one - but there's some curious context here. A few months ago, during a public exchange with leftist writer Matt Bruenig, Tanden plainly suggested that she was uninvolved with the law:
During subsequent coverage of the exchange, various columnists took her claim at face value. Kevin Drum, for example, insisted that it "is believable on its face", and Vox made the oddly tangential point that Tanden "was finishing up law school in 1996, not running welfare policy in the White House". As far as I can tell, the truth of the matter probably just comes down to litigating whatever narrow lawyerly definition of "involved" that Tanden has in mind. One doesn't have to say that she was "running welfare policy" to note that her name is all over contemporary documentation, which suggests that Bruenig's broad allegation that she was involved in the effort was substantively fair.
But all of this is just background for what happened next. Even though it turns out that Jilani was quoting Reed correctly, Tanden not only denied the claim, but - as she does - immediately used her platform to attack the reputation of a young journalist. Then, bizarrely, Tanden produced an "email" from Reed affirming her account - which was clearly just written in Notepad:
A few people have speculated that this was simply Tanden's way of hiding email headers, but that is beside the point.
People think Tanden's Notepad tweet is funny because they find her defense suspicious. Tanden has a history of denying her involvement in things that she appears to have been involved with, so one hesitates to take these kinds of denials at face value. It's also extremely odd that Reed would go on record making a categorical claim that he would instantly reverse after being contacted by Tanden, which makes people wonder if he was pressured into it. All of this plays into Tanden's reputation, fair or otherwise, as a person who is not always honest about her past, who leverages her power and influence to police her image on Twitter, and who is fairly inept with technology.
It also, of course, touches on the fact that Clinton and her campaign are currently fending off multiple email-related scandals. The mere possibility that a powerful member of Clinton's inner circle would try to pass off a fake Notepad screencap as an authentic piece of correspondence is hilarious precisely because it seems so in-character. My guess is that the theory that Tanden was simply trying to hide email headers is probably right - though that does nothing, of course, to undercut Jilani's quote, or to explain the documentation apparently implicating her in welfare reform, or to make the ongoing public relations disaster of Neera Tanden's Twitter account any less hilarious.