Page 18, footnote 7 of the government brief seems to explain this better
http://www.politico.com/f/?id=00000152-fae6-d7cd-af53-fafe53...
Edit- the title of this article is possibly misleading, it appears to be the iCloud password (not phone passcode) that was reset, which rules out the possibility of getting the phone to automatically back itself up while locked in a known wifi network.