The point of the story is
spoiler
The wallfacers have to have a secret plan that they cant share because the sophons can intervene if it is known. The Sophons are so powerful because they can interfere with any technology, only the human mind is beyond their reach. So he creates a technology to do exactly what he tested out that would achieve mutually assured destruction and the sophons cant do anything about it because... reasons.
It was obvious the entire time what he was planning and the result of that plan was MAD. If you come to destroy us, we will both die. He couldnt say it outloud because the premise was he was a wallfacer and there is no good reason this particular plan would suceed.