if it wasn't an exploit, and it just boiled down to player ingenuity taking battle content designers by surprise, yoshi would have just commented on how surprised they were by player strategy.

however, what actually happened was yoshi himself acknowledged a design flaw in garuda's AI in calculating HP before using aerial blast. we can prove this two easy ways: first, there's no way to skip astral flow in moogle or ifrit fights. second, we can see examples of garuda's calculation of remaining HP influencing battle mechanics both in pushing her down fast enough that she skips second plumes phase and goes straight to the third, and in pushing her down fast enough that she skips phase 2 after aerial blast. perhaps you haven't experienced the latter since you're blm burning it

the fact that being successful with blm burn strategy requires working around her HP calculation check for 2nd plumes so that you can skip an obviously required element of the fight (aerial blast) is exploitative, no matter how you slice it.

i'm pretty sure that they have never in the history of this game hotfixed content just to make it more difficult. considering how many casual players play ffxiv, doing something like that would cause a shitstorm.

contagion bug @ ifrit? exploit. hotfixed.
DoL wrist flick stunlock @ ifrit? exploit. hotfixed.
die/reraise hate reset strat @ moogle? exploit. hotfixed.

and now, there's this. so.