This is the less commonly heard anti-phrase, of the
popular " the cake is a lie"
suggesting that the said thing is true, or factual.
The "cake is not a lie" can also refer to the realasing of the
game "portal 2" which has not happened yet.
smarter man: the cake is a lie!
smarter smarter man: the cake is not a lie, the cake is delicious.
Tyrone: "Excuse me good sir, I've completed 'Portal' and let me tell you, the cake is not a lie."