The elderly Orc told Eiro, Bavet and Armodeus all that he knew about the Monster King. There wasn't much, to be honest. But Eiro didn't expect that much either. He just figured that he would know more than Eiro himself.
Eiro was created to be a mindless drone, a pure meat-shield that probably wouldn't be able to hope to even kill a child when going all out.
This Orc, on the other hand, was created in order to be a leader. To conserve knowledge that was meant to bring the village to the point where the Monster King wanted it to be. Hence, he must have been born with a lot of intelligence, enough to catch on to certain details that even Eiro was unable to figure out using his 'Memory of a Scholar'.