"Rustoleum" is used very loosely in forums such as this, rarely specifying which of the many Rustoleum-brand products is being recommended (or complained about). The most common Rustoleum products are just spray paint and have no specific rust conversion or rust-inhibiting ingredient. However, their Rusty Metal Primer is designed to bond to iron rust and does so, but I don't think it has any phosphoric acid or similar "converter" ingredient. In general, Rustoleum products "stop rust" simply by sealing the surface to stop moisture and air, the two things needed for rust to form, from reaching the metal. Any quality paint can do the same.
Rustoleum Rust Reformer claims to be a "converter" but I don't see the chemical identified anywhere. It says only that it's a vinyl acrylic resin. Gotta be something more, but what?