Hi wildegeist. Actually, the rpm-ostree command will install an RPM package of the application. This is what’s known as package layering, and is another way to add software to Silverblue. It creates a new ostree image that is a combination of the base ostree image plus the installed layered RPMs. Installing layered RPM packages requires a reboot before you can launch them.
However, Flatpaks are installed using the flatpak install <flatpak-app-name> command or Gnome Software app and are located outside of the ostree image. Flatpak installs do not need a reboot after installation to be able to use the application.