There's certain criteria you have to meet in order to be called a professional or have your job called a profession
OK I read the link. Appears that it's not valid to say Indie development (or in fact ANY computer game development, even AAA games) is a "profession", it can merely only be called an "ocupation" haha. However, I believe it is valid to say that someone does something in a "professional manner", be they Indie developer, AAA game developer, or toilet cleaner, or killer - because it's just a comparison when used like that.