Did you read it? That first paragraph’s last sentence refers you to the section which tells you how to opt out.

L. 30-Day Right to Opt Out. You have the right to opt out of arbitration by sending written notice of your decision to opt out to the following address by mail: General Counsel, Roku Inc., 1701 Junction Court, Suite 100, San Jose, CA 95112 within 30 days of you first becoming subject to these Dispute Resolution Terms. Such notice must include the name of each person opting out and contact information for each such person, the specific product models, software, or services used that are at issue, the email address that you used to set up your Roku account (if you have one), and, if applicable, a copy of your purchase receipt. For clarity, opt-out notices submitted via any method other than mail (including email) will not be effective. If you send timely written notice containing the required information in accordance with this Section 1(L), then neither party will be required to arbitrate the Claims between them.