I am afraid they omitted "?" at

  type Partial<T> = {
      [K in keyof T]: T[K]
  }

in their announce. It should be

  type Partial<T> = {
      [K in keyof T]?: T[K]
  }

to make

  interface Thing {
      foo: string;
      bar: number;
      [baz]: boolean;
  }

  type PartialThing = Partial<Thing>;

equivalent to

  interface PartialThing {
      foo?: string;
      bar?: number;
      [baz]?: boolean;
  }