That's incorrect. Those statistics include all the MERGE commits from libav into FFmpeg, which happen several time per day. Michael did not do 1500 commits more than the second commiter.
Michael Niedermayer is an EXTREMELY prolific dev who has been banging out FFMPEG code for more than a decade. It's by no means a one-man show, many other devs make large and valuable contributions - but in relative terms Niedermayer absolutely dominates FFMPEG, both in terms of personality and code output.