Well, it isn't in itself a "true oki finisher", it is "specific combo's true oki finisher"
In this game, as you hit your opponent more and more, you break down the "proration" further and further, and all of your subsequent attacks do less and less damage and have less and less untechable time, meaning for some combos when you finish with 5B 2B jB jC, they will be stuck til they hit the ground, and others, they won't be.