How do I configure Gmail Sync Accounts for iOS post January 2013?
No, I wouldn't say you're missing anything. As you say, Google will continue to support any configured devices beyond January 31, 2013 (from http://googleblog.blogspot.ch/2012/12/winter-cleaning.html):


  Starting January 30, 2013, consumers won't be able to set up new
  devices using Google Sync; however, existing Google Sync connections
  will continue to function. Google Sync will continue to be fully
  supported for Google Apps for Business, Government and Education.
  Users of those products are unaffected by this announcement.


so no changes for your devices whatsoever (from http://support.google.com/a/bin/answer.py?hl=en&amp;answer=2716936):


  What do I need to do if I’m already using Google Sync?
  
  Nothing! Existing users can continue to use Google Sync on their
  current devices.
  
  What will happen if I keep using Google Sync after January 30, 2013?
  
  The service will continue to work for existing Google Sync devices.
  Google Sync will continue to work, be supported, and take new sign-ups
  for Google Apps for Business, Education, and Government customers.


For new devices, or if you wish to stop using Google Sync on your devices, check these links:


CalDAV for iOS devices: http://support.google.com/calendar/bin/answer.py?hl=en&amp;answer=151674
CardDAV for iOS devices: http://support.google.com/mail/bin/answer.py?hl=en&amp;answer=2753077




As a side note: Windows Phone users, on the contrary, got an extension and Google will accept new device connections until July 31, 2013 (from http://support.google.com/a/bin/answer.py?hl=en&amp;answer=2716936): 



See also this blog post.