Yeah. The bigger problem, correct me if I'm wrong, is that the original western Steam version had Japanese language and subtitle options, but SE patched them out, leading to this DLC.
So yeah, if you bought the JP version on Steam, there'd be no problem. This is for Japanese people who own a western copy of the game.
Region locking incoming next gen!
Grey importing is becoming a big problem for certain countries, it's half the reason the UK's game sales are so high, because all of Europe is buying their games in the UK because they are half the price after the exchange rate.