I don't think fluke and unsustainable mean the same things. Both squads were full of players that were absolutely capable of playing to the level that they did. Both performed great under new managers and when their backs were up against the wall. It's not unlike the Cardinals in 2006 and 2011 in baseball. The Pujols era had some great regular season teams that won 100+ win seasons, but some of the weaker teams with those players happened to win the titles.
Fluke indicates that they weren't deserved or earned. Like a weird scenario where all the top seeds got elminated early, and a mid team ends up being the best team remaining. Or some choke job by the refs that ruined the game. Or some other factor that makes the titles not legitimate.