Visvanath,
Correct, that is why the term concurrent users is problematic to begin with and why I use requests by distinct users per second. Concurrency should be measured by number of concurrent threads/processes, http connections, db connections, and disk IO. Requests by distinct users per second is an easy to gather proxy for those stats.
You can get at concurrent requests from Moodle logs if you know the average time for a request to complete.