Maybe the lack of parallelism in OCaml's threads at the time he wrote that?

There are now at least two solutions to obtain speedups on multi-core and multi-processor machines plus scalability by allowing seamless distributed processing: the JoCaml extension, which integrates the join calculus (http://jocaml.inria.fr/), and coThreads (http://cothreads.sourceforge.net/), which comprises shared-memory (with extensions like STM) and message passing while keeping backwards-compatibility with the original Threads library.