Technically true, but FOSS isn’t “free” in the sense that someone is contributing labor to build and maintain the software. Free to use, but not free to make. I personally wouldn’t expect or shame a person for using FOSS without contributing. But if you make a profitable business off a FOSS project, it seems reasonable to expect some form of contribution back to the project - not because it is technically required, but because who better to sponsor a project than someone profiting from it?
Turns out that owning the platform (Android, iOS) counts for a lot. I like having an independent option.