Can you plagiarize a fact- or evidence-based, objective conclusion/claim?
To answer this question, one need look no further than the dictionary, which defines “plagiarism” as


  the practice of taking someone else’s work or ideas and passing them off as one’s own


What you are describing absolutely fits this definition. You read ideas that were published by another researcher some years ago, tying together some historical facts and drawing conclusions from them about Sultan Suleiman. Now you want to republish those exact same ideas in your own name (with immaterial differences in which sources you rely on to justify the exact same claims), without citing the source or even the fact that you read these ideas in an unnamed source you are now unable to locate.

Sure sounds like “passing off someone else’s ideas as your own” to me. So yes, of course it would be plagiarism. All those rationalizations about “not the author’s own voice”, “objective conclusions” and so forth, are just that - rationalizations. If you want to publish a paper, find something original to say, and don’t mislead your readers about the provenance of your ideas.