It's fair to say if you've heard of a historian that's usually the case. Most historians write one, two significant books in their life, if they write more it's almost always on the same topic over and over again. Their articles are always on the same topics. Ferguson appears to have actually hit these standards before he shifted to popular history.
This is actually why many of these guys are often so convincingly good, they've done the real work once in their life, so they can pretty well fake it on multiple subjects. You can see it compared to someone like (since he's on this page and I don't have to search for a better name) Molyneux who is essentially a complete amateur despite having a grad school desk in the university library. But generally if you "know" of a historian and they write lots of books, regularly, on many different topics*, they've basically "left" academia with their works, even if they keep their "day job" in academia. This has actually become increasingly common in history because of its different publishing standards. The best way to make money and keep your own interest for decades is to establish in academia and then just publish outside it.
Also, most historians in academia will focus on one area and even though they will teach lots of areas because they're "best at the school" they'll rarely actually present works outside of their specific fields. Ferguson for example appears to have his root expertise in British foreign policy history around 1914 and specifically British economic history regarding the turn of the century. He then extrapolates above that into general economic history and British history and then above that to his general popular histories about the West or whatever. He would, had he remained purely in academia have continued to only write about the British foreign policy decision making in 1914 and British economic histories of different types around that early 20th century period, with some backwards into the Gilded Age stuff to build the history. Like his Rothchilds book goes backwards to 1849 since that's when the family starts becoming relevant. He would teach general British history and economic history courses. He may teach history courses on other topics because often these can be entirely interest based but he would never for example publish the Kissinger or any of his American history books, it is so far outside of his expertise.
That's not to say that people can't switch focuses in history or even produce such a book, but it's highly unlikely as it would require simply too much work to shift appropriately. If anything you would expect him to partner with someone who "knows" Kissinger to approach British foreign policy for World War I either from a Kissingerist POV or how it is reflected in Kissinger's foreign policy. To write a two-part biography of the guy though off his notes? Never.
To be clear, I actually don't have a problem with popular history. Just wanted to build off what jake said.
*David McCullough, Doris Kearns Goodwin, etc.
