[Russian accent]"Aye K'pt'n, raising deflectors ..."
[emotionless]"No effect against our shields, captain. Sensors show they are using facts. We should be safe for the moment..."
You don't know if the game is based around the array, 12, or some other number. You are making assumptions and nothing more.
Mearls didn't ever come right out and say there were going to be 'crystal clear guidance' about which stat-generation method the game was balanced for, the way he did encounters/day.
But, the standard array is really close to the ranked average of 4d6, and point-buy can build that array, so, it seems like 'balancing around' any one of them would be as balanced as possible for each of the others.
It's just that random would give you more variance around that baseline than point-buy which'd give you more variance than array - all of which are likely dwarfed by other sources of imbalance, anyway, of course.
Rolling stats quite literally cannot break the game.
I've said it before: 5e is not a pane of glass, it's a pile of sand. Same substance, the one breaks easily into sharp shards and can't readily be repaired, the other just gets scattered about a bit and is easily restored to a neat pile.
Yeah, the DM is essentially a janitor in that analogy, I suppose...