For example, to implement a simple "hello world" program in Rust you have to use a macro (println!), so you can't even look for a function signature in the standard library docs to help. So you can either just say "don't worry about this for now, just trust me" or spend a whole chapter diving into macro syntax. The number of concepts you need to implement a basic program is pretty large and you could easily spend a chapter going into any of them.
Personally I'm not a fan of the approach in this post to just "lie" to people but I do find myself showing a non-optimal implementation because that's all the syntax I've introduced up to that point. Then later I show how to do it better. I know some readers just want the final answer up front though.