I think that person B had a bit of a rethink. Initially he said that he had slept on it enough (ie, yes, he had come to a decision). Then he realised that actually in coming to that decision he hadn't slept very much at all so he said "Or [perhaps I should say] lack of sleep!"
It's actually very badly worded because saying "I've slept enough" where sleep is used as a verb, and then following it with "lack of sleep" where it's used as a noun doesn't really work. It would have been better if he had said "I've had enough sleep [to come to a decision]. Or rather I've had enough lack of sleep!"
In order to make some kind of almost-pun on "to sleep on it" he would have to have said something like "I've slept on it enough. Or should I say "I've not slept on it!"?"