It all depends on your point of view. Making and selling mobile apps could be a lot more than a hobby.
Contributing to a FOSS project - is that a hobby? I'd say the number of hours a week put in determine that.
I got to sit and have a long conversation once with Louise Solheim. Her husband was Karsten Solheim. He had passed away by the time we met, but she told my wife and I the story of how he started his business and transitioned from being an engineer to selling golf clubs. A lot of what he did for a very long time could easily be described as a hobby - but it became an incredibly profitable business over time.