that's a good guess. I would agree. SPI is not connection-oriented and "comes up and goes down" (to the extent that it goes up and comes down at all) very fast with very little active memory needed to support the communication.
Neither is PS/2, which also being a synchronous serial interface with very low overhead, does share many characteristics with SPI at the physical layer:
The PS/2 interface saves power due to its interrupt-driven nature compared to USB which requires periodic polling, so it is a popular interface for laptops' internal keyboards and pointing devices
The major difference is that PS/2 is a de-facto standard up to the application layer/software interface, while the MacBook SPI protocol is unique to Apple.
