By his own account, he had help. But he claims to be the one doing the leg-work. He also has been developing contacts in proper political blogging for at least a couple of years, and in this day and age, that raises profiles. The leak could easily be a blog-reader.
I can also tell you, from personal experience, that "freelancer" is not necessarily the disadvantage you may think,
because the definition of the role of a freelancer is that you're free from the constraints of direct editorial oversight, and precisely because you have contacts all over the place, both inside a variety of publications, and can select the best outlet to place the story. A staffer can't.
The vast bulk of my work was in the tech sector, not politics, and even I had invitations to meetings and even dinner with a variety of billionaire CEO's, and unsolicited invitations to Downing Street meetings. A 'connected' freelsnce blogger, 19 or otherwise, is,
IMHO, an entirely credible option for an insider wanting to "leak" and that youth, and the implied naivety, may be part of the reason.
Did he get this on his own? Dunno, as I wasn't there.
But is it credible? Oh, yeah. It is.