The point is that it's the same kind of attack. Relying on the BIOS may save you from an attack on the disk firmware but that doesn't much help if the same class of attack is still effective against the BIOS.
I'd bet most systems see different disk controllers more often than they see different BIOS chips. I'd bet (though not at so high odds) that reasonably secure TPM chips are relatively easier to find outside of the high-end server niche. I'd bet that most non-state actors executing this sort of attack wouldn't have equivalent exploits ready for many different types of hardware.
All of those factors shift risk around (again, what little risk there is from this sort of vulnerability). Forgetting about patching a hole here because of an equal-sized hole over there is silly.