You're right. NED's Synclavier I (1977) was on the market before the Yamaha GS1, but unfortunately for them, Yamaha had already licensed the patent four years earlier (1973).

Check out how complex the design of the Synclavier is: http://www.studiorepair.com/gallery/misc/NED_Synclavier_II/ (and this is the more modern Synclavier II!) It's huge, and cost a fortune. I'd totally understand a company not wanting to manufacture and market it.