Although Paul Kennedy has trained as a professional historian, his work is kind of a synthesis of history, economics, and international relations theory. So he is a person who is bringing ideas from a few different social sciences together rather than a person who is focusing on history.

A more professional historian would probably focus more on narratives specific to a time and place and the evidence for or against those narratives. Kennedy is taking the work that other historians (and economic historians) have done in specific areas and stitching them together to create a bit of a quilt that provides an argument for a specific theory of international relations.

His work has generally been well received academically, especially by IR departments. However, it's 37 years old now, so it wouldn't totally surprise me if there were some examples Kennedy uses where the historical narratives he cited aren't considered as credible as they used to be. But that's not necessarily a bad thing, and it's almost inevitable if a book is still relevant two generations later.