He's saying that your intention is to use this tool to rename a file, in-place, but the implementation of this tool is `mv` which is supposed to be for "moving"

He's saying that your intention is to use this tool to rename a file, in-place, but the implementation of this tool is `mv` which is supposed to be for "moving" files around in the filesystem, so the implementation is leaking into and clashing with the user interaction.