I think the "secret" is a collection of songs he's been working on since Life Is Good. Why? He has been secretive about whatever that project was since it was announced. Nas had a title and everything ready to go in 2013. He even went on Twitter to announce it. His management was saying the project was crazier than Life Is Good. Then nothing. He appeared on Schoolboy Q's "Studio" and hinted it was done and the single was dropping. Then nothing. We got "Nas Album Done" and then nothing. In the midst of all that Nas would even tell interviewers not to ask about the new album. He did talk about a bit about what he was working when Khaled interviewed him.
Then we got Nasir. Even when that dropped, he talked about he had music with RZA and Swizz. Then we got Lost Tapes 2 which featured two RZA tracks and two Swizz joints, but we don't know if there was more.
And here we are with his latest and best of his recent work in King's Disease. In the midst of it dropping, we were treated to a snippet of a song he was doing with Dre. We already had muted footage and a picture of him in the studio with Dre.