XZ or not doesn't matter here. Even if you have a completely uncompressed tar file, if you only have one copy of it and lose access to that copy (whether bitrot, or disaster, or software error) then you've lost the data.
If I have a copy on my NAS, on a local backup disk, and in GCS, then there's no plausible risk to that data. I could go further and put another copy into AWS Glacier, or write it to tape and store it at my bank. At enterprise price points there's vendors like Iron Mountain who will store tapes by the container-load.
To claim that multiple copies is insufficient is absurd.
Copies are distinct copies. If your RAID catches fire, you want a copy that's somewhere else. Think external drive.
In terms of backups, while you might want redundancy for availability, you want distinct copies in separate places ideally. So that if your building catches fire and takes your RAID, at least you have a copy somewhere else.
A lot of these copies aren't in real time. That's what makes them an easy backup solution. A backup is a snapshot that allows you to go back to a point in time and see/recover things. Redundancy won't protect you against someone deleting their home folder. If the copies are in real time, that's gone too.
So, copies aren't always backups and a backup isn't always a copy.