I'm pretty sure it's still blockable but I wouldn't be surprised if it wasn't during flailback animation for SNES, it actually can be just be:
Harpoon, LP, Harpoon, LP etc so if you could somehow get 4 juggled jabs before a harpoon so that the 6th would make them flail back, it might then become a real inf but I doubt it.
The Harpoon in SNES UMK3 is like MK3. There are a lot of residual programming errors like Jax's missile spamming. Regardless it's an inf.