If you examine the files, you'll find that Das Boot data wasn't ADDED to SH4 for future use - all those files were left over from SH3. In other words they didn't program SH4 from scratch, they took SH3 and reprogrammed it to "create" SH4. So what we actually got was an SH3 super mod called Silent Hunter IV.
As for the stats, in this link;
http://uboat.net/fates/sub-sunk.htm
You have to click on each U-Boat individually, what I see is some Dutch, Norwegian, but mostly British subs sinking all these U-Boats, only two - both in the Java Sea - sunk by American subs. The actual number should be 23 rather than 24, because U-18 was damaged in a Russian air raid then scuttled in the Black Sea:
"on 20 August 1944.
The wreck was raised by the Soviets in late 1944 and declared a total loss. Sunk on
26 May 1947 by a torpedo from the Soviet submarine M-120 off Sevastopol."
To my mind that doesn't count as a WWII sinking by another submarine, in reality it was a cold war test of what would later become the fast attack submarines designed specifically to hunt and kill enemy subs.