The terraforming options are a bit arcane. Basically, many planets have limited terraforming options because they're too hot. Orbital mirrors allows you to overcome any such limitations; however, the problem with Inferno worlds isn't that they're too hot, it's that they're covered in a dense layer of corrosive gases. (You may notice that the in-game temperature report for an Inferno world is sometimes, actually "Warm" rather than "Hot" -- and that's because the Inferno type isn't actually limited to Hot worlds).

As damurxac says, the main barrier to terraforming Inferno worlds is the atmosphere, and the only way to deal with that is atmospheric condensers -- which is an Orthin specific tech.