Hey,
it seems at https://download.owncloud.org/download/repositories/production/owncloud/ only SLES is listed there. But when opening https://download.owncloud.org/download/repositories/production/ it seems at least OpenSUSE LEAP 15.1 is showing up there.
There was a short discussion around this repositories in Signature verification failed error on openSUSE repository, maybe you also need to ask the repository maintainer at the github bugtracker for the status of a Tumbleweed repository?