With sensor fusion you can improve the accuracy over just using GPS and accelerometers; it is quite common in quadcopters.

altitude is usually the least accurate position given by a GPS. A barometer helps you get closer, faster.

I have some MPX5100 pressure sensors connected up to an Arduino. When I first experimented with them, I was shocked to find that they were accurate enough to report pressure change simply from lifting the device off my desk and up to eye level.

Not that this says anything about the sensor used in the iPhone, but it's not unreasonable to expect that it could be used as some form of motion input.