He does a lot of pre-production preparation. That's not the same as having the knowledge otherwise. It's one reason why he's often criticised; he doesn't always know when he's simplifying and skipping over important aspects of the periods he describes. His listeners also won't know unless they happen to be particularly well read in that given area.

By saying he's not a historian, he frees himself from the accountability to have better fact checking. If he screws up, he can just say he's not a historian. Fans of history aren't held to the same standards as historians.

One of the biggest differences between a fan of history and a historian is the sheer number of hours put into reading up on your topic. A PhD in history is not a prerequisite for being a historian. A masters degree isn't either. But if you've just gotten your masters degree yesterday, you've already done a lot more reading and development of critical reading skills and interpreting of primary and secondary sources than the average person, even someone like Carlin. I think people without graduate degrees underestimate the number of hours that degree holders have spent dealing with their field. I say this as someone who was guilty of thinking so. It was only after I finished a masters that I realised all the hours behind me.

A degree is not what makes you a historian. I would say what makes you a historian is the time spent, the literature read and the development of the appropriate analytical skills.

As an under grad history major I am finding out the road is a long one.

Not necessarily.

I think the answer is more or less the same as the one to the question "what makes a government legitimate?" If enough of the "right" people recognize you as being a historian, you are one. The easiest way for that to happen is obviously through formal education.

But there are other cases of people without fancy graduate degrees who have obtained that status. Producing scholarship of a high enough quality (and being fortunate enough to have the right people read it) can certainly be enough. Richard Frank only has a law degree but I don't think anyone will dismiss him as not being a historian after his book Downfall.

In the case of Dan Carlin. I think there are a few reasons why he doesn't claim to be a historian: modesty; as a way of deflecting criticism; and because he essentially just presents things he's read rather than producing original scholarship himself (not that synthesis of existing scholarship is necessarily devoid of originality, of course).