If that leakage only brings the probability of failure back to previous rates, then you've still lowered the failure rate overall.

No, that's not what would happen. Helium drives enable higher data density, because helium decreases the amount of drag, which in turn reduces turbulence, platter vibration, and heat. This enables more more platters per drive and more accurate head positioning.

Helium leakage would make the drive substantially less reliable than air drives due to the higher data density.

But they're not comparing denser helium-filled drives to less-dense air-filled drives. They're comparing 12TB helium drives to 12TB air drives—equal density. (I have no idea how they're managing to make non-helium drives of that density, but apparently they can.)

Of course, for a drive density that can only be achieved using helium, a helium leak will kill the drive.

But right now, it seems that these drives might not actually need the helium, since equivalent drives exist without it.

FWIW a quick search suggests the 8TB drives have the same RPM too, so it's not like the helium-drives are running faster just because they can. (Helium-filled HUH728080ALE600 versus air-filled ST8000DM002.)

Technically, you could choose a 3.5 inch, 6 Gb/s, 4 Tb model that contains He and then compare to the non-He version.