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?
I use rclone to mount the Linux NAS from my Linux and Windows computers - SFTP backend is usually fine. Then I am uniformly reading/writing the NAS files as the local NAS user.