Most middle and high end soldering irons have had digital control loops run by microcontrollers for decades and that has serious advantages over a "dumb" iron (though there are some analog closed loop systems like Weller). The targets for IronOS are mostly small portable irons with compact screens and controls that have to be run by something, so at least you can run open source firmware on them.

Edit: Though to be fair, I think the popular ones already have open source firmware from the manufacturer but I may be mis-remembering.