Interesting, because I would have interpreted it as "I've slept on that" meaning I forgot it or haven't been paying attention to it.
To sleep on something ("Let me sleep on it and I'll get back to you") means to consider it attentively. The phrase that would better fit your interpretation is "I haven't sleep on that."
ETA: As I typed that, I realized that there's another sense of sleeping on something that means the opposite: "Don't sleep on Sammy Watkins." This just proves that the people who
invented the English language were, by and large, morons.