Yeah, when the Arceus was distributed for Entree Forest, B2W2 didn't even exist for players.
(GF was probably already developing it, but that does not matter in this case)
And GF/TPCi never re-hosted the event when B2W2 got released.
So it's not for B2W2. As a matter of fact, Gen 5 doesn't have the PIDIV correlation, so if you change that Arceus to BW, it'll show up as legal on PKHeX.
But the fact of the matter is: it's obtained in B2W2, so hacked.