200k XP in 15min was never intended otherwise all fates would potentially be as rewarding. The spawn rates for Fates are extremely accelerated. You could not generate that same amount of exp doing chains on level 50 mobs. Comparing the two is idiotic. No other source of experience even compares in the game. An example of a FATE that massive amounts of kills are intended is the "Return of the King" FATE in multiple zones.. The massive amounts of Mobs spawn but they well below the intended level range for the zone and only provide small amounts of Exp. There is also a limit to how many can be killed unlike the lvl 49 Fate in question. Things spawn as fast as they can be killed until the timer ends.

People are taking advantage of a situation that should not exist therefore meeting the exact definition of an exploit.