The design behind Plan 9 predates both Alef and the threading library. Lots of fruitful things were developed on Plan 9, and some folded back into the low-level systems. libtask was developed for Alef and only made available to run-of-the-mill C application later. According to Wikipedia, Plan 9 began in the mid '80s, and libtask was folded into the mix in 2000, two years before the project officially ended.
Limbo succeeded Alef, and Go seems to have succeeded Limbo. But I'll admit this is all just largely opinion.
But more to the point, Plan 9 never touted intraprocess message passing. If you read any of the papers, when they talked about parallelism they spoke about their rfork() system call (which may have inspired Linux' clone syscall; OpenBSD's rfork is clearly derivative of Plan 9). Alef, and later Limbo, was where all the experimentation into messaging passing went, and I think my comparison to Awk v. Unix is apt. To see where I'm coming from, just read their own description and emphasis: http://www.cs.bell-labs.com/sys/doc/9.html