There are quite a few FOSS projects that have developers who are paid to work on them. The biggest is Linux. But to claim that the vast majority are? I’d say that’s wrong.
Sure, but you somehow have to weight the size and/or importance of the project as well. If you count in all the not-updated-in-the-past-5-years 50-line projects on github, yes you might find that hobbyist coders are behind the largest number of FOSS projects, but that's a rather silly conclusion.
