Processes and threads on NT are distinct nominative types of objects (in a system where “object” has a much more precise meaning) and the GP is at least correct that the former are not schedulable entities. This distinction doesn’t really exist on Linux for instance where there are at one approximation on the user side only processes (at least to use the verbiage of the clone syscall - look elsewhere and they’re threads in part due to having to support pthreads), and the scheduler schedules “tasks” (task_struct) (whereas in NT the “thread” nomenclature carries throughout). FreeBSD may have separate thread and proc internally but this is more an implementation detail. I guess this all to say at the level lower than an API like pthreads, process/thread really isn’t easily comparable between NT and most Unixes.

It’s not so much “heavyweight” vs “lightweight” but that NT has been by design more limited in how you can create new virtual memory spaces.

For better or worse NT tied the creation of VM spaces to this relatively expensive object to create which has made emulating Unix like behavior historically a pain in the ass.