It would seem that he saw
Robert L Holt in the afternoon and shadowed her until night, and made a surprise attack after the moon had set. It is possible that the crew of the
Holt never actually had a target to shoot at.
http://www.u-boot-archiv-cuxhaven.de...rt_l_holt.html
I wasn't aware that there was an actual order against engaging with guns. I would think it was a matter of the fact that one hit on a submarine can render it no longer a submarine, but just a slow torpedo boat.