Ok, my take is this. the launch codes are generated by a computer that is hooked up to another computer, blah blah, that is connected to the interwebs and can be accessed by some mega hacking by an alien born artificial intelligence.
Even if they are not generated on such a computer (though, they probably are), the codes pass through some machines with memory at some point, even if it is only on the way to get printed. And, unless such machines are *truly* isolated (which includes having *no* wireless access at all, anywhere in the network) then it may be possible to reach the codes, or the algorithm and seed that generated them.
And, if Ultron can suborn or threaten someone into introducing a wireless connection to the isolated network... even an internet-capable toaster...