It was far from clear at the time that "Amok Time" was written that Vulcans were "co-founders" of anything. We had never seen a Vulcan other than Spock and had no clear idea whether anyone else on the Enterprise had, either.
I know that's hard to imagine forty-odd years later, but Vulcans were rather mysterious and exotic (to use a somewhat politically incorrect but useful word) and little if anything had been said about any alien members of the Federation other
than Vulcans. Sturgeon was an imaginative science fiction writer and he approached the Vulcans as if they were an alien species, rather than simply a national or ethnic grouping (the latter being the way Trek has chosen for the most part to treat alien life since) who lived on an isolated planet. He thought in sf terms, IOW, rather than TV.