I think it has something to do with the size of the ground wire used. In my case I used all (4) individual conductors the same size. BUT I think code here allows a different smaller size ground wire, but requires a ground rod.Yeah I was wondering about the ground rod. But no one around here does that so I guess it's not required. It must depend on the distance to the main panel (total resistance).
The way I looked at it, was if I ran all the same size wires, the remote panel would be no different that a sub panel in the basement, which does not require a ground rod here.
Pretty sure you will be fine. Depending on what size that apparently aluminum ground wire is that comes in which is part of your UF cable. I ran mine separately, with three individual conductors, in conduit. All four wires were copper of the same size.
Still, if I remember, this year I am going to put the ground rod in...can't hurt.