The method is balanced, because every player can choose the same point spread if they want to. The results between player might be not balanced, but on the other hand players with classes that need to be decent in a lot of stats and players who can dump a lot can do that and don't have to take the fixed array. In that sense its even more balanced than a fixed array.
But overall it doesnt matter really, there is no perfect method of stat distribution, just take one that feels good. For me personally a fixed array is boring and bland, I prefer rolling or at least point buy.