How'd he get so many stars?

If you file your short film with the SCREEN ACTORS GUILD, you can use names (union actors) that agree to work for little or no money, as long as you follow SAG's strict work conditions/requirements and have production insurance. If a star likes the project, they might be interested in doing it for little or no money because of the short time commitment.

That's what I'm guessing happend with this film.