The wiki is about upgrading a 4.x installation from Jewel to Luminous only (this one does not apply to a 5.x installation).
https://pve.proxmox.com/wiki/Ceph_Jewel_to_Luminous
If you upgrade from 4.x to 5.x, this article contains the details, including the needed change in the repositories.
https://pve.proxmox.com/wiki/Upgrade_from_4.x_to_5.0#Add_the_PVE_repositories_to_you_installation
All in all upgrading a Ceph installation is possible, but you need to read exactly the howtos and you have to follow exactly all steps. Best practice, test the process on a test cluster (e.g. in a virtual ceph cluster).
https://pve.proxmox.com/wiki/Ceph_Jewel_to_Luminous
If you upgrade from 4.x to 5.x, this article contains the details, including the needed change in the repositories.
https://pve.proxmox.com/wiki/Upgrade_from_4.x_to_5.0#Add_the_PVE_repositories_to_you_installation
All in all upgrading a Ceph installation is possible, but you need to read exactly the howtos and you have to follow exactly all steps. Best practice, test the process on a test cluster (e.g. in a virtual ceph cluster).