That still doesn't address the original post. How do we know that the films were sourced from 1080i/p sources? Just because a tape can hold 1080p material doesn't mean that it does.
Just look at how Warner Brothers is sending out 720 and 1080i broadcast masters (or even downloads) too stations of the first four seasons of "Smallville", even though they have stated that the material isn't up to Blu-Ray standards. It could be that Paramount was in the same boat, but instead of saying that the sources weren't up to Blu-Ray specs Paramount just unpressed a 720p tape too 1080p and added some film grain too make it look like they had taken the Blu-Rays from the film.