If you look at the background Casey was in, then his thought process becomes a bit understandable. He worked for a long time doing R&D at RAD Game Tools, which is a company that provided various game development middleware and tools. So his interests naturally gear more towards low-level game tools/systems development rather than high-level design and scripting, which is why he couldn't understand the value proposition of more widely-used monolithic game engines like Unity, Gamemaker, etc. (For example, his attitude towards actual gameplay code in Handmade Hero is "It's not my concern, I'll finish the engine and then someone else will fill the gaps").

Note that he almost did have experience in creating an actual full game by himself before, though he didn't publish it eventually since he was unsatisfied with the final product, which is quite absurd to me! (https://www.destructoid.com/he-worked-on-it-for-three-years-...) He also worked on The Witness team with Jonathan Blow and others, but mainly on engine programming. He is currently working on a new game called 1935 (along with his other myriad projects like Handmade Hero and Star Code Galaxy), but details about it are very, very sparse.