I have spoken to a friend who was part of the team who created this the first year. He has since left the company, so he can't comment on how they do it now.
But he said it used to be the number of segments only, and you need to have flown at least 5 or 6 diferent *A airlines.
The "last" one who still made it into the Top1000 only had 38 segments or so.