It may just be a mis-communication. Someone at the lawyer's office may have sent off the subpoenas not knowing that the case. But, remittitur is not a method of settlement, and it may actually mean that the case has been remanded for further proceedings.
You should contact an attorney to look into the matter. You don't want to violate the subpoena if it's legit.
If the case is closed the subpoena is invalid. If the case is still open the attorney can issue a subpoena regarding any open issues. You say it appears the case is settled with remittitur and the information you have from opposing counsel is that the case is closed. Further you made a statement that there are private negoiations. I cannot confirm if there are any open issues.
A remittitur is a ruling by a judge (usually upon motion to reduce or throw out a jury verdict) lowering the amount of damages granted by a jury in a civil case. Usually, this is because the amount awarded exceeded the amount demanded. The term is sometimes used for a reduction in awarded damages even when the amount awarded did not exceed the amount demanded, but is otherwise considered excessive.
If the motion is granted, the plaintiff may either accept the reduced verdict or submit to a new trial.
So there may still be some open issues and the case may not be entirely closed.
Even if the case is closed you may need to serve an objection to the subpoena. You need to serve this objection timely so you should probably do it right away so you do not waive any rights.
You could also pay an attorney to write the objection at a very nominal cost. One phone call could clear up the matter up.
It would not be proper to issue a subpoena on a closed case.
Let me know if you need anything else. We offer a free 1/2 phone consulation.
You should see what you can find online about the particular case. If it is a state court case, the online status may be a little unclear, but you may be able to determine if the case is truly closed as to all parties from the last few entries in the register of actions or case summary. In federal court, you will need to use PACER to access the case activity and it will usually state the status of the case prominently, but that is not always accurate.
Also, simply because some parties are in the process of settling, that does not preclude the attorney from continuing to subpoena witnesses and conduct discovery. If all the parties have settled and the settlement is now on the record, then further discovery attempts could be a problem, but I would not assume that the subpoena is invalid to compel you to provide the documents, merely because you have been advised that the parties are having settlement talks. That does not sound to me as though this case is closed.