The standard for defamation is:

. knew that the statement was false and defamatory, or

. acted with reckless disregard of the truth or falsity of the statement in making the statement, or

. acted negligently in failing to ascertain whether the statement was true or false before making it.

Not vetting your sources can be seen as acting in reckless disregard of the truth or acting negligently in failing to ascertain whether the statements were true.

Of course, the story was posted on his blog. That's 100% his duty, or at least if he wants to maintain his reputation as a journalist.

Considering he considers his work "independent investigative journalism", I'd say that it is.