The problem is that being an arbiter of truth and a presenter of truth were once not very different things. That is, simply presenting facts was the key to both. More specifically, once those facts were presented there was no need for arbitration.
So, if there were two sides in a story and, say, an investgative journalist unearthed and presented some fact that resolved the matter, then you might loosely say that journalist was an arbiter of truth on behalf of the public by simply presenting the fact. This was true even as little as a decade ago. And it might have even been considered pedantic to distinguish between journalist as presenter or arbiter.
But this is no longer the case. We are now in an age where people actively refute facts and deny reality itself. They sell the idea that the audience can choose what's true. So when the same journalist presents a fact, he is then accused of bias and becomes a part of the story. Thus, even simple truths have now become debatable to the point that merely presenting them can be cast as arbitration, as if these facts are subjective, thus there is still some matter to be decided.
So, if there were two sides in a story and, say, an investgative journalist unearthed and presented some fact that resolved the matter, then you might loosely say that journalist was an arbiter of truth on behalf of the public by simply presenting the fact. This was true even as little as a decade ago. And it might have even been considered pedantic to distinguish between journalist as presenter or arbiter.
But this is no longer the case. We are now in an age where people actively refute facts and deny reality itself. They sell the idea that the audience can choose what's true. So when the same journalist presents a fact, he is then accused of bias and becomes a part of the story. Thus, even simple truths have now become debatable to the point that merely presenting them can be cast as arbitration, as if these facts are subjective, thus there is still some matter to be decided.