The Swizz/Nas album was done in 2016. From what's been said of it, it seems like something that just happened.
Then there's the album he's been working on since 2012. There's several different sessions:
2012- he started working with Timbaland
2013- Timbaland confirms "Sinatra In The Sands"
2014- Nas revealed work with Tim, Swizz, and No ID. There were also sessions with Reefa and Mike Will
2016- Kahled alluded to the Nas/Swizz album by saying he'd been hearing talk in the streets of Nas and Swizz having some crazy records. Nas confirmed saying he and Swizz had a Virgo moment. In the same interview, Nas talked about sessions with Large Professor and Da Beatminerz. He also said Khaled had been feeding him beats from Cool & Dre.
2018- Nas drops Nasir produced by Kanye and co. He then reveals he's still working on the long awaited proper follow-up to Life Is Good and confirms Swizz and RZA as producers
Pretty much Swizz has been the common thread in the whole process. Just looking at pictures in this thread over the past few years, Nas and Swizz have done a lot together from sharing birthdays to appearances at events.