I took a peek at the code and it looks like their visibility timeout is pretty much a lock on a message. So it's not exactly once for any meaningful definition, but it does prevent the same message from being consumed multiple times within the visibility timeout.
... When there is no failure of the underlying system. The expectation of any queue is that messages are only delivered once. But that's not what's interesting: what matters is what happens when there's a system failure: either the message gets delivered more than once, the message gets delivered zero times, or a little of column A and a little of column B (which is the worst of both worlds and is a bug). If you have one queue node, it can fail and lose your data. If you have multiple queue nodes, you can have a network partition. In all cases, it's possible to not know whether the message was processed or not at some point