One reason he is an absolute fool is that the clergyman had trained him from the beginning. Scoresby was not the smartest man in the world. The clergyman was his teacher at a military academy at Woolwich. He says Scoresby did not know anything. Scoresby was so sweet and lovable that he couldn’t see him fail. He then started to teach Scoresby everything he needed to know about Caesar and mathematics. When the test time came he passed and beat the smartest people in the class. Without the clergyman’s help Scoresby would have not passed the test and would not have moved on.
Another reason Scoresby is a fool is that during the war he was given orders to fall back. Instead he moved forward and found the Russian enemy sitting around, not expecting that someone will attack them. When Scoresby found them he attacked and all the Russians ran away in fear. People may have died and if he listened there was less of a chance of people dying.
The most important reason is that he is lucky for being the best. During the war he was getting an In-Battle promotion. He is lucky that the clergyman was nice enough to help him at the academy which got him a higher rank. Due to that he was able to rank up during the battle much faster. He is also lucky to find the Russian army not ready for an attack.