Their explanation. How you respond, if at all, depends on what the notice says. If it's phrased as a "demand" which your bank is required to satisfy, my guess is that they are simply full of it.
Any jwrsp account you hold passed to the survivor at the moment of death. They say that does not defeat their claim, but that's between them and the new owner.
Any account that was solely in the name of the decedent is now under the sole dominion of a duly appointed personal representative. You are in no position to evaluate and satisfy claims against the decedent's estate.
Note, the government makes mistakes. If you "satisfy" an errant claim with money that actually belongs to someone else, my guess is that your bank will become liable for that decision, just as you should. Either way, you have consulted with your legal counsel, take his or her advice.