Yes, as far as I know Gotoh is the only supplier of the Phase III tuners and as much as I like Schaller hardware that's a clever decision.
If I recall, PRS also made a deal with Robson and Gotoh to make the tuning machines now used for the Private Stock acoustics, that were formerly made by Robson in England. There appear to be small differences in the machines, but having had both the Robson and the Gotoh/PRS/Robson models, I can't tell any difference in operation.
I wonder if Robson had a hand in the open back Phase III tuner design?