Spoiler:He had a whole story. Not just "Oh don't worry about it"
Mr Nobody has access to hard light holograms in 2021Spoiler:No, he had a story about what he was doing during the time between 3 and 9. They had no story for how he faked his death besides "Mr Nobody is good at making people look dead". They literally handwaved the part on HOW he faked his death. And he says "don't worry about it" to Sean when he asks how he's alive.