Looks like the Pokemon Shuffle community just can't get away from issues with hackers. Once again, we're seeing the leaderboards dominated by people hacking the game and making the impossible possible.
Right now, all of the teams on the North American Mega Absol Competition Leaderboard feature the same team of Ditto (unobtainable & unreleased), SHINY Genesect (unobtainable & unreleased), Genesect & Xerneas. The top score of 641,490 is posted by a player named “Jesus”.
It's unclear if Nintendo/Pokemon Company are working on this issue. They've taken care of some problems in the past, but issues just keep coming back. Thanks to Anon for the heads up.