How to verify disk hit by a query
The numbers reported by log_statement_stats come directly from the getrusage system call (assuming your OS has one).  The numbers in square brackets are total for the session so far, the other numbers are deltas between the start and stop of the statement.  

The filesystem block size is not normalized to be the same as the PostgresSQL block size.  It is in whatever units of block size your OS reports in getrusage.