From the same book, Uncle Al states at the end that Earth is a very dangerous place. Since all the Goosebumps books take place in the same universe according to Word of God, he's absolutely right.
Billy is not only slightly more headstrong and tough than the average protagonist, but he's also better at handling wild animals, river rescues, and firearms. Since he's the child of two military scientists under a covert space program, this is probably not a coincidence.
Billy mentions animals like bears and snakes, even though they would not be present on an alien planet like his. How did he know about them? Because he heard about them from his parents’ expeditions.
