Since you don't need to be a member in order to read the threads, that figure doesn't include all the lurkers. Maybe the 10K user figure isn't so far off the mark.
Of course, a different question altogether is "how many of us are still using amiga hardware?" In my case, I have a PC at home that I only use for web browsing and an accelerated A1200 that gets used once or twice a week to play classics like Syndicate or Beneath a Steel Sky. PC games suck these days... all 3D eyecandy and no gameplay. I really hope the next generation of amiga games writers won't fall for the same trap.