I see it as they had the mantle driver portion of the driver ready but they've had many iterations of drivers in between. Now when BF4 is ready, they integrate the mantle portion and got some issues. They say installation issues, not driver issues, so that's what it appears more like to me...
That's the thing to understand. It's not giving the card MORE work to do, it's allowing the card to do a greater amount of work in the same time through optimizations. A pseudo example:
Pretend the card has to do this now:
1. Load texture A into buffer
2. Load a filter for use
3...