It's a backup with a retention policy. In backblaze's case, it's 30 days. I'm fairly certain that still counts as a backup. Do you think that only immutable, append-only storage counts as "backup"?

That's really what the personal 'backups' are, it's just a synced copy that will happily mirror corrupted data and delete files that were deleted (not sure if there's any delay). If you want real backups then use the B2 storage directly.