"Of course you're right, multiple AZs can fail at the same time, but I read the above as saying that they should fail independently/coincidentally."

As far as I know we've heard nothing to the contrary from Amazon - it's totally possible that multiple AZs happened to fail independently/coincidentally. Perhaps it was simultaneous equipment failure? Or maybe one AZ failed and a sufficient number of people attempted to "fail over" to another AZ causing a chain reaction of failure?