To expand on laulis' comment: Valetudo isn't a full custom-firmware, it's a mod for the existing firmware. You copy on the Valetudo daemon binary, fuss with the init scripts to start the daemon, and fuss with the DNS and such to point some domains at 127.0.0.1 to talk to that daemon instead of the normal servers (well, actually you probably download a firmware image from dustbin that already has those modifications applied).

This is a distinction that is worth making because the robot is still running and relying on all of the on-robot proprietary code; it's just the in-cloud code that has been replaced.