The urea injection thing is used on coal plants too, scrubbing the nox emissions. Since air is mostly nitrogen, using air as the source of combustion oxygen pumps nitrogen thru the engine, which makes nitrogen oxides. These are very bad environmentally, and the def fluid is used as a scrubbing liquid. Gad scrubbers are very important in industry for other stuff too, so the technologies are very well established and known. Making it compact enough to work on a truck or excavator was a learning curve and engineering challenge, and if you are around one they smell funny, but they are far less smoky and far quieter.