That's because he comes from a domain where mutable state is a necessary thing in making the different parts of game logic feed into each other. It is the focus

That's because he comes from a domain where mutable state is a necessary thing in making the different parts of game logic feed into each other. It is the focus of debugging. The actual computations around what changed can be pure, but turning them into a concurrently operating loop is done most effectively with a static sequencing(and hence, straightline imperative). Or in other terms, he is using a different strategy for different parts of the codebase. At the top level it's imperative, but when you drill down into the callstack, it isn't. A lot of folks work on request-response systems all day, and in those, the necessary state changes tend to be once-per-request and wrapped in transaction or session logic. So it's a lot less crucial to have this kind of strategy for debugging purposes.