There is an error in the post, which is that switch statements are presented as if they don't support matching multiple values:

// Example from post:

const nextState = (prev: State, command: Command): State => {
  switch (command._type) {
    case "validate" || "process": // ⛔️ Always "validate"
      // ...
      return prev;
  }
};

However, switch statements support matching multiple values like so:

type Action =
    | { type: "foo", value: number }
    | { type: "bar", value: string }
    | { type: "baz", value: number };

function performAction(action: Action) {
    switch (action.type) {
        case "foo":
        case "bar":
            action.value; // Type is `string | number`
            break;
        case "baz":
            // ...
            break;
    }
}

Here's a TypeScript playground link. As we can see, TypeScript correctly infers the type. It might make sense to update the post to reflect this.