Thanks! I did not know this.
However, I find myself thinking that chemically true is one thing, and best emergency measure may be another, and very different.
If I find my tank in this situation, the first 2 things I would do, are - 1) water changes to remove the nitrite, and 2) identify and fix whatever is wrong with the biological filter, and add established bio material.
I would definitely NOT go and add salt as a first measure, even knowing that chloride ion is absorbed in preference to nitrite ion.
Thus, the concluding statement of this may be chemically true, but does not jive with my thinking of the best immediate solution-
("Keeping the chloride levels in the water at least 20 mg/1 can prevent nitrite toxicosis").
Instead, the second part of the statement is what I would aim to always do first - ("Additional treatments can include emergency water changes to dilute the nitrite problem") if having such problems.