I guess what they’re saying is, even though it’s “not supported” officially, you can still try and there’s good chances it’ll work anyway. If you need or prefer to stick to a supported configuration, it seems your options are either to switch to podman and figure out nextcloud, or switch away from RHEL.
I believe the point is, once some data is publicly available, even if you try to delete it, you can never be sure all copies are truly gone. Like you said, maybe it lives on somebody’s hard drive, maybe some other user managed to scrape it for their own personal use, maybe they screenshotted the most compromising posts, etc. You can never be sure it’s gone.