You can search for his account and find him, but the search "from:mtaibbi", which should show you all his tweets, gives 0 results. That's the claim being made and it's true.
The claim is that no posts by Matt Taibi show up on Twitter search. And that is true. No posts made by Matt Taibbi can be found via Twitter search. Yes, his profile comes up and you can click through to it. But his posts themselves are not indexed by search in the usual way. If you want to find a post Taibbi made months ago, for example, good luck doing so.