I think its the issues they are seeing here https://forums.swift.org/t/differentiable-programming-for-gr...

Until the video of the last meeting is posted im guessing maybe its bc the execs set a deadline and it got passed

It's kind of odd. Chris had a huge impact at Apple with LLVM and Swift still going strong and forming the backbone of the developer tools. But I'm not sure he has anything of lasting value to show for his jobs since then. Maybe evidence that for great engineers, just like other great people, fortuitous timing and circumstances are a big part of the story.