They did also say there will be a new tutorial which will start characters off in Morrowind instead of the base game maps. But I assume that will be optional - like you currently choose an alliance and choose whether to play the Wailing Prison or skip it you'll be able to choose whether to start in Morrowind or not.
Well apparently Morrowind will be on the PTS at some point so I assume we'll find out then, if it doesn't get explained before.
I can't see them forcing you to start in Morrowind if you have the expansion. Yes I know you can just use a wayshrine as soon as you arrive but I can guarantee some people will be furious that their immersion is ruined by having to do that and will claim they can never start a new character because of it so making it a choice seems like the obvious thing to do.