[catechu]

While I do not know whether or not Watson actually does this, the paper doesn't mention anything about parallelism -- I suspect that the exploitation of structured parallelism is another handy benefit to using Prolog on as many machines as Watson was using.

(For example: http://lambda-the-ultimate.org/node/1867.)

[microtonal]

There is so much ambiguity in language that parallelization is often possible at a very course-grained level. This usually doesn't require more cleverness than multi-processing or multi-threading.

Given that they switched from their own pattern matching language to an optimized WAM, I suspect they use one of the common Prolog implementations.