Disclaimer: the following contains a fair dose of speculation.
The TMS is really a human computer interface between the driver/fitter/tester and the "real" electronics under the hood, which are almost certainly all ASICs designed to guarantee that operation occurs within safe bounds to a greater degree than an RTOS would. The power/brake handle is wired up to these circuits directly, which operate using the parameters the TMS has set previously.
The only difference between new stock and slightly older stock is that the TMS replaces the classical banks of switches, dials, and buttons and that the ASICs replace the larger PCBs. Older stock still used relays.