Final answer:
The Sibyl's mention of individuals who found out knowledge surprises Aeneas in the underworld.
Step-by-step explanation:
The statement from Sibyl that surprises Aeneas is when she mentions 'those who found out knowledge and used it for the betterment of life' (663). This takes Aeneas by surprise because he encounters not only mythological heroes and famous war heroes in the underworld, but also individuals who have contributed to the advancement of knowledge in various fields.
This suggests that the afterlife rewards not only military valor, but also intellectual achievements.