This construct works fine for Storeman Installer (and was reused for the SailfishOS:Chum GUI Installer), after performing some research, which is also documented in its GitHub repository. You may want to take a look there.
P.S. (rather to @YouAll, than you personally):
Do not use the rpm command for anything else than the rpm -q sub-command!
Do use pkcon for managing RPM packages on your SailfishOS installation, or alternatively zypper after installing it by executing devel-su pkcon install zypper