I am pretty sure that PostgreSQL uses thread from version 8.0.

That is not true. There is an auxiliary thread used for signal handling on Windows (which is somewhat hacky), but the basic model of one process per client connection is unchanged, and applies to Postgres on both Windows and Unix.

the tuning documentation for PostgreSQL *states* that good IO performance has more of an impact than good CPU performance

This is often, but not always the case -- it depends on the nature of the query workload. In this case, the benchmark was done using read-only queries, specifically for the purpose of minimizing the I/O requirements and therefore focusing on CPU performance.