Binding arbitration’s legal
It is almost certain that the letter is simply restating the dispute resolution terms in the contract that you agreed to when you opened the account.
If it isn't in the original agreement then they are proposing a change - you can either agree to this change or, as the letter suggests opt-out of the change. Or you can close your account.
Binding arbitration does not offend the 7th amendment because while your right to a trial can’t be taken from you, you are free to voluntarily give up that right in a contract. As you did.
Also, the 7th amendment is one of the few that doesn’t apply to state law - it’s likely that a controversy with a credit union is a state matter.