After a little more research, it seems that he is playing some of the stuff. In that clip though, he is 'trumpet-syncing'. He says he been learning to play for 3 years, with that little skill I would surprised if he really is playing everything in the movie.
I wonder how much they'll really delve into Miles, too. He was quite polarizing.
Here's a quick story that a trumpet teacher told me about Miles:
When Miles was a freshman at Julliard, he studied with William Vachiano, long time principal trumpet player of the New York Philharmonic. Bill wanted to work on transposition with Miles so he pulled out the 3rd trumpet part to Strauss' Ein Heldenleben. Bill says, "This excerpt is in the key of Eb, your trumpet is in the key of Bb, where does that leave you?" Miles retorted, "That leaves my trumpet in the case." Miles then walked out of the lesson.