I'm not aware what they used before, but why pthreads? Isn't this perfect opportunity to try to develop something better, introduce new concepts (async, coroutines or some kind of userspace deterministic job management)? Not sure if backward compatibility is best way to go here, but maybe i'm just talking gibberish
I agree that raw threads are a terrible abstraction for application programmers to manipulate directly. Actors and CSP are much better concurrency models for application developers to use.
However, if you're going to do low-level OS and language runtime programming for multi-core computers, threads are a good abstraction on top of which you'd implement CSP or Actors. Also, there's tons of legacy software that uses lots of raw pthread calls, so to have any kind of adoption of a UNIX-like OS, you're going to have to support pthreads.