Although I remain somewhat sceptical over the broader accuracy of any official pronouncements regarding DDG-1000 (they will be correct word-for-word, at the time released, however), it is not impossible to imagine that the 30mm would be more preferable for certain scenarios than the 57mm, I'm specifically thinking about multiple small boats approaching from a wide range of directions. Although the 57mm is undoubtedly more capable at mashing up a single boat, it may become overwhelmed at a lower number of threats than the smaller calibre weapon. And that comment about RPG range being about a mile annoys me. Offhand I think RPG-7 rounds self destruct at about half that.
Regarding the Millennium gun - it's an above deck mount which would require sailors to expose themselves to enemy fire whilst reloading, and limits the number of engagements. It would require additional upperdeck equipment to allow personnel to reload, which has been avoided in the interests of reduced RCS. There are also issues with it's actual lethality against boat targets - it makes a lot of holes but it's not certain that they will be lethal - remember that it's original target set was fast moving missiles and aircraft, whose own KE is a weapon to be used against them.
RP1