Coroutines are light (cooperative) threads, not preemptive ones, and Lua is strictly single-threaded too^. Lua Lanes library provides hardware threads to Lua programs by creating separate vm states and managing interactions between these: http://lualanes.github.io/lanes/ but it is another beast.

^ except rare cases when embedded with lua_lock() defined as a thread locking routine. Then it becomes thread-safe, but still not multithreaded.