He's been talking about it for at least seven years now. Same title, same concept. He used to post concept art and mockups every once in a while but hasn't for a few years, until now apparently. He's given previews of the lore and story and his thoughts on realistic technical abilities and how that will determine the graphics and overall aesthetic. There was also a tech demo released for a different game that he was making just to test the waters of game development.
The last time he talked about it he basically said it's going to be really expensive and time consuming but it's his dream project and he wants to put in the time to do it right. It's going to be a big game. He said he expected it to take at least ten years and he started streaming to raise funds specifically for the development. Then the boxing match happened which lead to a new wave of popularity and relaunched his career to a much bigger audience so he's making a lot more money now.
Pretty sure he's been continuously working on it in some capacity for several years and maybe now he's looking to spend more time on it. It will be released. It might be horrible and take three decades but it will be released. It's obvious by the way he talks about it that it's his passion project. Keep in mind he first started talking about what would become Fishtank eight years ago.