I think the others have already answered your question, but perhaps what was confusing is that there seems to be a word missing: "It could just as easily *have* been off by $12 billion."

It is common for English speakers when someone is criticising errors, to emphasise the error by doubling the error. Just like Jimmy has explained. The regulators missed $6 billion, but they are so incompetent it could or may be more we do not know.

The misstatement was actually $6bn, but nobody noticed. The last sentence states that there would have been an equal probability that nobody would have noticed if the misstatement had been twice as big. I think this is an example of hyperbole: the choice of the number 12bn is almost certainly arbitrary, the writer is simply using it to stress the incompetence of the regulators.

If your question is regarding the sentence "It could just as easily been off by $12 billion."...then it means the earnings were misstated by $12 billions instead of the stated $6 billions. This last sentence is actually assuming that it is more than what was identified.