What does zero downtime mean? No interruptions of services? Or just that people could still log in all the time?
Pretty sure they mean people could login again after the switch. I can't imagine what the purpose would be of capturing session data for each logged in user and transferring that over... I wouldn't even expect that of a fortune 500 company moving platforms. If that is what they did, it warrants a post on its own.
It means users' puzzle and game progress was never interrupted, along with login sessions. A half-played puzzle before the cutover could be picked up as it was afterward.