They're implemented (in my mind so far) as preemptive threads, one per interpreter; which makes them slightly more heavy-weight than Erlang's NxM and a nice complement to purely cooperative multitasking.
It's one of those languages that does things differently to solve actual issues, not to check boxes.
From my limited experience, Erlang doesn't share data between processes; you throw it over the fence by sending to the process inbox, which is where the locking takes place.
Still, shuffling data between OS threads is an easier problem to solve than serializing between OS processes.
They're implemented (in my mind so far) as preemptive threads, one per interpreter; which makes them slightly more heavy-weight than Erlang's NxM and a nice complement to purely cooperative multitasking.