The content model for <steps> requires at least one <step>, and the content model for <step> requires at least one <cmd>. This true even when using @conref or @conkeyref. Try this:
The problem that you are having is exactly why I don't like the fact that the DITA content models require elements. This sort of restriction belongs in a Schematron that can be set up to not throw errors when @conref or @conkeyref are used.