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.