As a former ASW guy I can assure you that we will be able to detect and destroy these boats if it becomes necessary.
We have technology available for dealing with exactly this type of threat.
D/E boats do have a low amplitude signature because they do not have to continually run reactor coolant pumps, but they are NOT undetectable.
I have heard these brags before. There once was a gunnery officer on a certain surface ship who made the brag we never got close enough to their ship to shoot a torpedo at them. Our captain gave them a picture of their ship taken by our periscope. It took 3 pictures to get the whole ship shown because they were so close. Sure you can detect them but sometimes you can't find them. Thermo layers and other evasive devices often make the task very hard.