The article claims that the steel rusting is actually an integral part of the process. The iron in the steel reacts with CO2 to rust, which sets the material.

I actually read the carbon negative claim as implying they're forming an iron carbonate. I'm not sure why it would be carbon negative if it's just iron to rust, rust doesn't have any carbon in it.

Watching the YouTube video (mentioned further down) which covers in-development concrete alternatives (https://youtu.be/wOE4UegzJ_M?t=190), apparently this forms FeCO3 + H2.

The C in that would be the carbon sequestration. :)