Yes, you are correct, I was mixing engine programmers that both worked on "The Witness". Casey is known (among many other tools and research) for Handmade Hero[0], an engine he uses to teach very low level game programming concepts. Which he is using to release a full game with (and you can "pre-order" the game on the website which gives you access to the source code)

0: https://handmadehero.org/