maybe they were backing up their stuff properly, but backups were wiped as well. even if you have some fancy append-only storage someone still has access to it and that access can be misued.
They could have wiped through other means, e.g. through ipmi. Although I don't think that was the case.
More realistically, it probably boils down to money. I wonder what would be the cost of backing up everything to a competitor's cloud daily, e.g. one PB of data per day. I have no idea how much it even costs to have a 200 gigabit link to another data center.
I believe this is the case of "no true Scotsman". Whatever backup you propose, someone will point out that you could have done it better. You could have disconnected servers with backups from network when they are not in use. You could have hired a dedicated person whose responsibility would be to deny/delay any request from management to delete the backups. And so on.
Then they're not offline backups, are they? I know what you mean but backing up to a network drive with R/W is not a backup, it's a copy.