You get the idea. Ultimately, isn't a professional historian kind of an oxymoron? There can be a professional economist/military officer/lawyer, who applies their skills to history, but to itself, a historian is not a profession
A paleonthologist, anthropologist, archeologist etc. all have specific skills. They're scientists who can use scientific method (carbon dating etc) to a certain aspect of the past to speak of it with a measure of authority. A historian to me is about as much of a scientist as a priest or a politician. Maybe even less.
Which isn't to say that a credential makes a person automatically better at something, but as a general rule of thumb - yes, it does. I would not go to a mechanic to cure my cold and a therapist to fix my car. So why should I trust some old man/woman about history if he/she does not have credentials to speak with confidence about this or that field?
Especially if it's a very multifaceted topic, like the entire history of an empire. If historian talks about tactics with which empire waged wars on one page, and then does an economic analysis on the other, and all they have to their name is a history degree and they don't cite people who have more fitting qualifications on each topic - then it's roughly as believable as LotR.