I think "Rank" is based on your "position", which is just about points. If you play long enough or a lot, your points should gradually creep up, so some old-timers may have a good rank and position even if their play is only so-so. The win and draw percentages are probably a better measure of the skill of a player.
An even better measure can be those ghost ratings, http://tournaments.webdiplomacy.net/theghost-ratingslist , which take into account the caliber of the opposition as well, so a player doesn't get too much credit for just beating up on noobs. However, the ghost ratings strongly favor aggressive play, giving a lot more credit for wins than draws, so they tend to downplay the skill of cooperative players in favor of the cutthroat ones. That's a fair way to measure skill in the game, but certainly slants it towards the cutthroat philosophy, which is not the only way to view the game.