You are right. It is not that they "cannot lose", but it is theirs to lose. They need go back only to 1988 to see where the party in power was in disfavor at this point - yet won in November.
(I use 1988, as there was no incumbent. As in 2000 - yet in 2000, the party in power did lose - so either is possible - and is why I say it is theirs to lose).