Arguably he shouldn't have had to, since the arbitration clause appeared in the Terms of Service after he made his pledge. I have my own opinions about arbitration but that's neither here nor there when what you agreed to did not dictate that you must resolve disputes through arbitration.

According to the article, he made a some of his pledges after the change to the Terms of Service.

I can imagine that if they tried to get a refund somewhat close to the change, a judge could be convinced to allow the refund under the old ToS.

But he has used the service since, making more pledges to other companies. Part of continuing to use kickstarter is to to accept the new TOS when they come out.

If he made one pledge, and then never used or logged into kickstarter and agreed to a new TOS then he would have a case.