the ldap backend is indeed the recommended backend for Kopano One, but it does not need to be ActiveDirectory.
Knowing that ldap is hard for most people we recently started a very simple go based project that provides the necessary ldap interface for Kopano. It can even be used as a drop in replacement (for Kopano relevant cases anyways). Its still in an early stage and therefore does not have more explicit documentation, but you can find the getting started information in https://stash.kopano.io/projects/KGOL/repos/kidm/browse. Its provided as a package alongside the nightly builds and in the Kopano One repository.