if it were true, then saltwater should never experience nitrites, it just simply isn't true
ahh..but all the hemoglobin molecules are not saturated with the sodium ion.
some still bind with the nitrite ion.
so, saltwater fish can experience nitrite poisoning just like freshwater fish.
However, it is true that nitrite is not quite as critical in a saltwater tank as it is in a freshwater tanks..and this is why.
Heck, my brackish tank (which is now full salt) took forever to cycle (using the fish in method). I had high nitrites for a month, but the fish were fine.
I kept thinking oh my god they are all going to die..but they never even noticed the nitrite levels of 5 mg/l.
I suspect this effect of 'competitive inhibition" (as the biologists say) is why.