Yes, while the armour in the opening trailer fits with many people's expectations of what Roman legions look like, it isn't accurate for the legions in Caesar's time. Unfortunately, for the pre-rendered images it's not really possible to change anything at this point. However, while some artistic liberties have still been taken, the in-game legions do wear armour more accurate to the time period.
Even after working on Rome for a couple of years, we expect that there are plenty of players out there who know the period a lot better than we do and so we're happy to be called out on anything you feel we've gotten wrong. We might not always be able to change it, but we'll do what we can.