Ex-Gimp User here. The article does point out that most photographer workflows are in their raw editors. You get Non Destructive Editing with tools like Darktable in Linux. Non destructive editing helps to improve your productivity. It's similar to making code changes knowing you have a revision system behind and to when you don't. That being said non destructive doesn't make you a better retoucher, it's there to save your ass, which happens less often for the seasoned professional. Professionals were productive long before non destructive editing. Gimp is used professionally, even though we hear little about that. Krita and Blender have all been used on big budget movies as well. Gimps biggest problem IMHO ( and strictly from my experience) is usability. This post isn't going to turn into a typical gimp ui rant even though gimp does makes my head hurt; a lot but I don't think I'll be going back to gimp unless I really have to. I have used since it since 2001 and seeing it lag behind other open source editors makes me sometimes wonder why usability is an after thought.