At least on Linux, processes and threads are not that much different. You create both with clone(2) call which accepts a large number of flags (see https://linux.die.net/man/2/clone ). Depending on the flags you pass, you can get a new thread, or a new process, or something inbetween. For example, how about a "thread" which has it's own memory space (~CLONE_VM)?
You cannot generalize scheduling primitives across UNIX implementations.
To quote the FreeBSD manual:
"Traditional UNIX® does not define any API nor implementation for threading, while POSIX® defines its threading API but the implementation is undefined."
macOS was at least temporarily considered a true UNIX, and the scheduling primitive there is a Mach task. I frankly don't remember much about FreeBSD anymore, but I would assume that the unit of scheduling there is somewhat identical to that of Linux... Just implemented nicer.
That is the whole point, Linux != UNIX, yet every time someone discuss some UNIX feature, there comes a Linux example as if Linux would the be representative of how UNIX works.
That's not what happened here. In this case, someone discussed Windows, Linux and macOS features and implementation details, and someone (you) came around and started talking about UNIX. ;)
