No, it's definitely a broken animation. Sometimes it kills. Sometimes it doesn't. And that is if the target is human. If it's Zin, its chances of killing outright is even less. One might say during the animation character slightly misses the heart but why even put it there if it's not gonna kill. I mean the point of super-powered beatdowns is that they have 100% chance of killing the target. Even without any investment into melee skill or namely, Super Strength. For one of the animations not to share that affinity and the fact that you can control it is a put-off. Because I really like beatdowns, super-powered or otherwise.
I also calculated that perhaps the exact number of hits delivered is related to the fatality of the move. Because you'll notice most SPBs are multi-hit combos. There are only two that are single hit, if I recall. One is Heart-Stopper (I mentioned in the OP), the other is where character grabs the target by the ankles, spins and throws, I call this one Wheelhouse. The latter is always deadly without fail. The former isn't.