Ultimately this is an argument about the precise semantics of "removable storage". I don't regard an MTP device as "removable storage" - it's another computer that one speaks to using a special protocol, with severe limitations. So is an iPhone - with special, protocol-speaking software, you can certainly put arbitrary files on it. I interpreted the parent to mean that they wanted to plug the phone in to "any PC" and have it Just Work. MTP isn't so great at that, especially from my perspective as a Linux user, where MTP support is no more built-in than iOS-protocol support. From this standpoint, "removable storage" == "USB mass storage".
On the other hand, it sounds like whatever Apple provides is much worse even than MTP, and less well supported in general.