Author bias or not, diesel-electric subs have worried the Navy for some time. See attached.
http://www.nationaldefensemagazine.org/issues/2004/Aug/Diesel_Submarines.htm
Anything in the worlds oceans ‘worry the Navy’.
This worry over diesel boats was the plot of Kelsey Grammer's silly movie Down Periscope. Where he was tasked with penetrating Norfolk with an unmodified WWII fleet boat, still carried deck guns. Silly as the movie was, the premise of alarm about the stealth of diesel electrics was and is real.