This. Keep in mind EU4 is built on top of Divine Wind and they werent exactly rolling out a new engine and they still alloted over a year of work to it. The game could be out tomorrow and after a couple patches it would be fine. I'm glad they took the extra time, but I strongly expect the game to be ready for early Q3 like Johan suggested.
Then again, there can be other considerations. Title releases have to be done strategically to maximize early sales. If its more likely to sell well in August, then they'll wait. But thats marketing stuff, not game design. To be honest though, I think its best to sell at the beginning of the summer rather than at the beginning of the school year. Kids have time off to play and adults tend to have vacations slotted there, surely that helps sales.