Mori far bought more than the SunTour name in 1990 - they bought SunTour's operations and facilities from Madea Industries, essentially by assuming SunTour's debts. Mori continued to produce both SR and SunTour products under the SR Suntour name for several years before deciding to get out of the bicycle components business entirely in early 1995. At that point, two of SR's former owners arranged a management buy-out of SR's operations and the SunTour name and the SunTour production facility in Taiwan from Mori and contined operations as SR SunTour.

Unfortunately, the new SR SunTour management team either chose not to purchase (or were perhaps unable to purchase) SunTour's intellectual property, other facilities, and other tooling as part of the 1995 buy-out. Berto says Mori sold SunTour's remaining tooling for scrap (which included the tooling for the Superbe groupset) and sold the rest of SunTour's remaining facilities individually. He doesn't say what happened to the IP rights.

I did some further checking, and it appears that SR SunTour produced products using the Hexon crank interface as late as the early 2010s. (Unfortunately, I've been unable to determine when it first appeared.) Based on that fact, I'd guess you're probably correct in thinking it was developed after the 1995 management-buy-out vice something developed by the old Madea SunTour development team.