Don't know where you got the above quote from but the problem was not testing in calm waters as the quote suggests - it was a TOTAL lack of real world "war shot" testing!!
It sounds incredible today that such a "revolutionary" device as the magnetic pistol was just assumed to be engineered correctly. It was the 30's - money was tight - and the design bureau knew best. Then, early in the war, there was a major torpedo shortage coupled with a lack of effective tactics and some timid skippers. On top of that you had some of the commanders in the Pacific who were directly involved in the development of said magnetic pistol. All of this combined to create a situation where thorough testing was resisted.