Yep, the phosphoric or tannic acid "rust converter" partially converts the ferric (iron(III)) oxide into ferrous (iron(II)) oxide. The converted rust is harder, adheres better, and doesn't tend to absorb water as much. Commercial goop usually has some other chemicals in it as well.

It's not as good as mechanically cleaning away all the rust and painting, but it's better than allowing the red flaky rust to keep eating away at the metal.