Can you plagiarize a fact- or evidence-based, objective conclusion/claim? Suppose a secondary source proves that poverty was rampant in the Middle Ages by using a variety of primary sources (or statistics from other secondary sources). If years after reading this secondary source I decide I want to make the same claim (that poverty was common), would it be plagiarism to make this exact same statement using the exact same sources (supposing I actually noted and remembered them)?

In other words, can you plagiarize an objective fact or claim when it's backed up by concrete facts, regardless of whether or not someone else has made the same claim?

If that example above is too vague, let me paraphrase with a problem that I have encountered recently. I once read a source that said that Sultan Suleiman (who led an attack on Vienna hundreds of years ago) had his good and his bad qualities because he was ruthless in war yet was very chivalrous domestically. If I can't find this source again, can I just find other sources that talk about his ruthlessness and his chivalry, and then make the same conclusion?

My personal opinion is that yes, this is totally fine, because these conclusions are not the author's own voice, but rather what evidence shows to be a fact. If a source tried to make the claim that Suleiman had bipolar disorder, then this would have to be cited since this is not a fact but rather a subjective interpretation. However, an identical piecing together of "microfacts" to create "macrofacts" does not seem like something that could be categorized as plagiarism.

Notwithstanding, I'd like some second thoughts on this. Am I correct?
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.