As a public figure, the bar to successfully win a defamation lawsuit requires proof of actual malice. This means that he has to prove by clear and convincing evidence that either the reporter knew the information was absolutely false, or had very good reason to suspect the information was false and still recklessly report it as fact. It’s a pretty high bar.

They weren’t making statements of fact as a witness themselves, there were people interviewed about things they claimed to witness themselves. Statements in the article that seems to have rattled him go like this:

But Patel, according to multiple current officials, as well as former officials who have stayed close to him, is deeply concerned that his job is in jeopardy. He has good reasons to think so—including some having to do with what witnesses described to me as bouts of excessive drinking.

and this:

“We’re all just waiting for the word” that Patel is officially out of the top job, an FBI official told me this week, and a former official told my colleague Jonathan Lemire that Patel was “rightly paranoid.” Senior members of the Trump administration are already discussing who might replace him, according to an administration official and two people close to the White House who were familiar with the conversations.

They also report Patels response to the claims:

The FBI responded with a statement, attributed to Patel: “Print it, all false, I’ll see you in court—bring your checkbook.”

I mean, it seems like they did a lot of interviewing, found a lot of sources, gave the opportunity for a response, etc. You know… good journalism. Unless the claim is that the Atlantic reporters made up these claims wholecloth or that they had good reason to distrust what seems to be a dozen plus sources from multiple government departments, it doesn’t seem like this lawsuit has a leg to stand on.