PhD in History here.

It's sadly not quite that simple. There are often paradigms that are difficult to shift. Disproving one theory/interpretation or showing how something was different than we previously understood can be fantastic for one Historians career, but st the same time it is detrimental to (typically) numerous Historians whose work has revolved around what is being "discredited." Thus, people can get very defensive over a given interpretation of the past.

Even in fields covering more recent history, such as mine, where it is widely understood that our understanding of the past will change repeatedly as new information is retrieved (personal papers being accessiblr after people die, old folks not caring anymore and spilling the beans, government document declassification, etc.), you still get some very... aggressive defense of one's work from people.

So, if you are looking at a peer reviewed journal, for example, you won't see this conflict from just taking a look from the outside. But if your article is going against the tide of the field's accepted interpretation of an event, behind the scenes you may have trouble. Your article is going to be sent out to two of the field's leading Historians to review. When they read your article basically saying that their past work is wrong, they will review your article poorly and tell the editor not to print it. The editors go off the reviewers, then you don't get printed.

On the outside, you only see articles being printed with new stuff in it, but you would never know that all those articles are bringing in new stuff that doesn't go drastically against the grain of leading Historians who are reviewing those articles.

This is the same process with university published books. It's really hard to get a high quality publication in general if you are going radically against the accepted narrative for these reasons, and thus, you don't get paradigm shifts often. It can sometimes take scholars retiring and a new generation who is less attached and defensive to become the new batch of senior scholars doing reviews.

Tldr - He isn't making that outlandish of a claim. Particularly in a field that has little hard evidence to go by, it's very difficult to shift the accepted interpretation of the past.