That's a semantic argument that isn't even accurate. Those pages and extents exist in files on disk and the disk controller must access them. If I rebuild the clustered index then the pages (and their contents) that were logically connected via page chains will be re-arranged (on the disk), i.e. an individual row may move from one physical disk sector of a file extent to another. If I insert a million new rows and my file group needs to extend then that additional disk space must be allocated. I'll grant you that the file system created the extent but it is SQL Server that is creating the physical pages that will be stored on the extent.

I suppose you could say that "technically" the only thing that writes to a disk are the read-write heads which are managed by the firmware of the device but that kind of defeats the purpose of the discussion. We don't need to be concerned with that level of abstraction. It's a lot simpler to say that SQL Server physically rearranges data on disk.