That they used external backups doesn't actually imply anything about the GCS backups being unavailable. And Google's press release explicitly notes that unisuper used both. (And there's all kinds of reasons to have used both, both good and bad)

Put formally, we have statements that

    - 1. A and B exist
    - 2. A was used
    - 3. A and B were used

Your conclusion from these statements is that, because (2) A was used, therefore B does not exist. Hopefully putting it like this makes it clear why I'm so confused.