Okay, technically it is something new – a picoprocess provider – because classic Windows environment subsystems were too heavyweight.
Trying to use the classical environment subsystem model (which POSIX and OS/2 used) for Linux wouldn't give you any clear advantage and would come with a significant performance cost. That's the whole reason why they added picoprocess providers to NT (originally intended to support Android emulation, later retargeted to generic Linux, the result of which was WSL1)
Okay, technically it is something new – a picoprocess provider – because classic Windows environment subsystems were too heavyweight.
Trying to use the classical environment subsystem model (which POSIX and OS/2 used) for Linux wouldn't give you any clear advantage and would come with a significant performance cost. That's the whole reason why they added picoprocess providers to NT (originally intended to support Android emulation, later retargeted to generic Linux, the result of which was WSL1)