You're right that OpenRC only replaces the layers above init, but that's where most of the interesting stuff happens. In a traditional sysvinit-based system, all the actual work of setting up the system, finding and running the correct init scripts, etc is done by shell scripts started by init. OpenRC replaces those shell scripts with a proper service management layer, with init's only jobs being to start OpenRC, spawn getty processes on terminals, and reap orphan processes.