Using the "Alternative Pattern" Mixin in TypeScript

I don't understand TypeScript's article on Alternative Pattern for mixins. I see that when I execute the code as they have on the website, I get the methods mixed in, but I don't get properties. e.g. this example

I added the property z = 1; to Jumpable, create my class, and console.log z, but the value is undefined. If instead I used the Constrained Mixins pattern, it works as expected, but I don't particularly care for how that looks and was hoping that I'm doing something wrong with the Alternative Pattern. Does anyone have any guidance?

Code for reference:

// Code copied from https://www.typescriptlang.org/docs/handbook/mixins.html#alternative-pattern

// Each mixin is a traditional ES class
class Jumpable {
  z = 1;
  jump() {
    console.log('Jumped');
  }
}

// Including the base
class Sprite {
  x = 0;
  y = 0;
}

// Then you create an interface which merges
// the expected mixins with the same name as your base
interface Sprite extends Jumpable {}
// Apply the mixins into the base class via
// the JS at runtime
applyMixins(Sprite, [Jumpable]);

let player = new Sprite();
console.log(player.z);

// This can live anywhere in your codebase:
function applyMixins(derivedCtor: any, constructors: any[]) {
  constructors.forEach((baseCtor) => {
    Object.getOwnPropertyNames(baseCtor.prototype).forEach((name) => {
      Object.defineProperty(
        derivedCtor.prototype,
        name,
        Object.getOwnPropertyDescriptor(baseCtor.prototype, name) ||
          Object.create(null)
      );
    });
  });
}