Yes, make a starter. You still need a decent amount of Sacc yeast.
As someone said, Brett works better when stressed (anaerobic rather than aerobic), so normally when using Brett as a secondary yeast, you would not make a starter.
In this case, WLP670 contains both a Sacc and a Brett yeast. The Sacc yeast works quicker than the Brett and will use up the majority of the oxygen before the Brett gets started, leaving the Brett very little to work with..