Cigna, for most policies, has a 180 day timely filing policy for appeals, based on date of initial claim payment. So while it looks like you are out of luck on the appeal front, I would contact your provider rep, explain the situation, and see if there is anything you/they can do at this point. Submitting a corrected claim probably won't work, due to timely filing issues also. This is an unusual situation, which is why you need a provider rep involved at this point.
My argument with the rep would be that the rendering provider was contracted with Cigna, even if the group was not, a huge mistake was made by the employee, claims were submitted incorrectly, and you would like them to be corrected at this time. Keep in mind if they do agree to do a mass readjustment on all the claims in question, they may ask for all the money back that was paid at the OON rate of 20%, before they reprocess at the in network rate.