According to the list I found here -
http://www.valoratsea.com/losses1.htm
Amberjack and Grampus were #10 and #11 of the 52 submarines lost in the war. So, allowing for the uncertainty inherent with submarine warfare the 7 mentioned in the article plus S-26 is not too far off. I wouldn't call coverup, all things considered.
Check the dates - Number 16 was lost yesterday. So they are understating by half.
Here’s the official list. They may not have known about number 15 yet. http://www.history.navy.mil/library/online/sublosses/sublosses_intro.htm#chron