What does "unauthorized attacks" mean in the iOS Security Guide?
Yes, if you authorize someone to "attack" your iOS device - that would be an authorized "attack".

This is not so far-fetched or unrealistic as it sounds. Many companies do purchase services from security vendors to do authorized "attacks" on their servers and devices in order to ensure that they have sufficient security. 

However, you're reading too much into the text if you think that it states that an authorized attack will succeed just because it is authorized. That doesn't make any sense.

So all in all, nothing new, nothing to be alarmed about, and nothing that is specific for iOS devices which is not true of any other device.