Earlier quoted context omitted.
It's a GO runtime setting. Has nothing to do with Syncthing.
I'm not a Go programmer, but couldn't they achieve the same thing without name collision by reading a custom environment variable at startup and setting runtime.GOMAXPROCS?
This setting is managed by Go runtime, and Go runtime defines a specific env var for customization. If you want different setting for different Go programs, no problem, it is not a global config, but per process, so set the env var to desired value only for syncthing process.