That's literally what determining facts means.

> They decide whether the evidence presented to them is sufficient to show that an event or events happened

Yes, they determine the facts—what is true and not—by weighing the evidence.

> My understanding is that even in defamation suits they are mostly trying to prove that person A said thing X about person C and then prove that person C was harmed because of it.

They also decide whether X is true, and, if so, whether Person A claimed it with knoweldge of it's falsity or reckless disregard of the truth. But the other determinations are also decisions about what is true.

And in almost any case, the jury is tasked with determining the credibility of witnesses.

In a legal sense, that's exactly what they do. Juries are, by definition, deciders of fact, i.e. they decided what is legally true based on the evidence presented to them.

Whether legal "truth," epistemological "truth," and physical "truth" coincide is an entirely separate (and largely pointless) issue.