[Palmik]

In ELO like match-making, you typically pair together people such that they are likely to have 50% chance to win. Therefore as the OP says, filtering down to people with high (60+%) life-time win-rate creates some sort of (interesting) bias.

I would select from all games played on sufficiently high level.

[pclmulqdq]

They don't fully use Elo for matchmaking. There's a league system, and you get matched with players in your league. The ranks reset frequently, too.

Edit - I did the math. From the data on the MTG Elo Project, top Magic players have about a 70-75% game win percentage over an average tournament player. They have the top player at ~2300 Elo with the average being around 1500 (in matches), and have scaled the Elo system so that a 200 point gap is a 60% chance to win a best-of-three match (this is NOT the same as Chess Elo scoring).