An exiled planeswalker returned with loyalty higher than printed in Arena
Your thinking is correct, when Teferi, Hero of Dominaria returns from exile as a result of Banishing Light being destroyed, it is a brand new object that should be exactly as if it was being played for the first time.

400.7. An object that moves from one zone to another becomes a new object with no memory of, or relation to, its previous existence.

There are several possible effects that could cause Teferi to have entered with 8 loyalty counters, but no way to know for sure what happened in your particular game unless you know the entire board state.
One simple possibility is that your opponent could have had Doubling Season or Vorinclex, Monstrous Raider or a similar card in play, which would cause their planeswalkers to enter with twice the normal amount of loyalty counters.
They also could have activated an ability such as Gilder Bairn as soon as Teferi entered play.