it appears that it would require ~33% more sodium sulfite than sodium metabisulfite to scavenge the same amount of oxygen.
If that's the case, it sounds like a jump in Na content, too.
I'd agree with you - good catch. Same two atoms of sodium per molecule, so increasing the dosage by 33% for oxygen scavenging purposes would increase the sodium 33% as well. However, I like that a possible alternative to SMB was brought up because it means we're starting to think more about the oxygen we let into our brew process on the homebrew level.
The idea of having utmost control over oxygen ingress into our brewing process is akin to starting with DI/RO water and controlling the mineral content we allow in. It's another knob to turn that can help us dial in some details that were previously unattainable. Maybe I desire some oxidized malt qualities in my barleywine, whereas I don't in my american light lagers - that kind of thing.