I agree with Grimace, ask for the bleeding procedure used, ask where in the service Manuel the procedure is described. There is no bleeder valve in the clutch hydraulics on 2011 and up mustangs.
OAC-Sparky: How can you vacuum bleed when there is no bleeder valve ? You vacuum bleed brakes at the caliper bleeder valve, why would you modify the master cylinder cap? A pressure bleeder is applied to the master cylinder not a vacuum bleeder, perhaps you are confusing the two, however I don't know how either can work without a bleeder valve. That dealer was not being honest .
You can bleed the brakes/clutch via the reservoir the same way you bleed a cooling system after a flush. When you apply a vacuum to the reservoir, trapped air gets sucked out the system(albeit slowly) and is actually the most thorough method for removing air. Because there are no collapsible rubber hoses and because the system is sealed on both ends, there is no flow of fluid during the process. What happens when a vacuum is applied is the air gets dissolved in the fluid then evaporates at the top of the reservoir. The same principle applies to the A/C system during a evac/recharge except the Freon is a gas at normal room temperature and pressure.