On the TOS: https://itch.io/docs/legal/terms#15-class-action-waiver

"YOU AND COMPANY AGREE NOT TO BRING OR PARTICIPATE IN A CLASS, COLLECTIVE, OR REPRESENTATIVE ACTION, PRIVATE ATTORNEY GENERAL ACTION OR COLLECTIVE ARBITRATION, EVEN IF ARBITRATION PROCEDURES OR RULES WOULD OTHERWISE ALLOW ONE. You and Company also agree not to seek to combine any action with any other action without the consent of all parties to this Agreement and all other actions.

If the agreement in this Section not to bring or participate in a class, collective or representative action, private attorney general action or collective arbitration should be found illegal or unenforceable, you and Company agree that it shall not be severable, that this entire Section shall be unenforceable and any claim or dispute would be resolved in court and not in a class or collective action."

There is nothing saying you can not sue them. Valve user Agreement has this cause: 

http://store.steampowered.com/subscriber_agreement/

"An arbitration is a proceeding before a neutral arbitrator, instead of before a judge or jury. Arbitration is less formal than a lawsuit in court, and provides more limited discovery. It follows different rules than court proceedings, and is subject to very limited review by courts. The arbitrator will issue a written decision and provide a statement of reasons if requested by either party. YOU UNDERSTAND THAT YOU AND VALVE ARE GIVING UP THE RIGHT TO SUE IN COURT AND TO HAVE A TRIAL BEFORE A JUDGE OR JURY."

And Legal books have known this since 1981:

n. a mini-trial, which may be for a lawsuit ready to go to trial, held in an attempt to avoid a court trial and conducted by a person or a panel of people who are not judges. The arbitration may be agreed to by the parties, may be required by a provision in a contract for settling disputes, or may be provided for under statute. To avoid clogged court calendars the parties often agree to have the matter determined by a panel such as one provided by the American Arbitration Association (which has a specific set of rules), a retired judge, some other respected lawyer, or some organization that provides these services. Usually contract-required arbitration may be converted into a legal judgment on petition to the court, unless some party has protested that there has been a gross injustice, collusion or fraud. Many states provide for mandatory arbitration of cases on a non-binding basis in the hope that these "mini-trials" by experienced attorneys will give the parties a clearer picture of the probable result and lead to acceptance of the arbitrator's decision. (See: arbitrator)

Copyright © 1981-2005 by Gerald N. Hill and Kathleen T. Hill.

What does this mean?

itch.io has a chance to have someone sue them if it breaks laws or unenforceable

While Valve says no you can not use us or bring us to court because you said yes to the User agreement and did agree you know such agreement.