It is opt-in, as it says right in the second paragraph and then again further down the page: If you would like the chance to help DeepMind with its research by

It is opt-in, as it says right in the second paragraph and then again further down the page: If you would like the chance to help DeepMind with its research by matching against AlphaStar, you can opt in by clicking the “opt-in” button on the in-game popup window.