FreeRTOS is a unikernel by that definition (by every definition I've seen, though they still use the term "Library OS" which seems essentially synonymous). It's quite common in embedded devices.
Well, the usual definition I see for unikernels also stipulates that there's only a single process. So FreeRTOS is still a unikernel, but early UNIX wouldn't be.
Though the distinction between "process" and "thread" gets very fuzzy when you have only a single address space.
Actually is the other way around, libc is the part of UNIX API ISO was willing to put into the language standard, while the remaining part of the C UNIX runtime ended up as POSIX.
