That's not quite my take. The transcript is ambiguous.
"Unknown" could mean "we have no idea who that was (even now)", or, "we don't know who that was,
at the time of the transmission".
But to me it's strange that someone unknown would be telling
RF2 specifically not to evacuate the plane, rather than asking
the pilots not to evacuate the plane.
So Capot's scenario seems more likely.
We're only seeing part of the picture here. "Various parties" have presumably interviewed RF2 in person, and others on the ground, to see what happened that day.
It's an easy question to ask: "hey RF2, was that you on the radio?"
This RF2 business is potentially a big weakness for the Captain's case in a jury trial. But as I mentioned before, I think Allegiant also made a series of mistakes (and have a huge
PR problem) so it's in everyone's interest to settle.