That's a surprising retcon to the Arrow
universe, but I think it's a promising development. Arrow
has turned out to be a strong interpretation of the DCU, so it could make sense to use it as the anchor for an extended DCU on television, given how much trouble they're having getting one to work in the movies. Okay, they are going forward with a Batman/Superman movie, but there are no guarantees.
Or, maybe the plan is to follow Marvel's lead and do a shared universe encompassing both film and TV. It's conceivable that Arrow
could be retroactively folded into the Man of Steel
universe (or vice-versa, depending on how you look at it).