Earlier quoted context omitted.
On the one hand you're saying it can't be done without some whistleblower turning up, on the other hand you think that its possible to happen accidentally. It's kinda either/or. It's either possible that someone can get full admin privileges and deletes it off-the-record or they've got enough safeguards in place that the accidental deletion isn't possible either.
It seems very reasonable that 47 million messages were deleted from 8,700 mail boxes by accident, but it seems like a stretch of the imagination to say this resulted in the intentional deletion of the EXACT evidence that they were being subpoenaed for, without forgetting to delete SOME of the evidence.
I don't think your argument works unless the article is wrong.