If it was a different Slave Marker than his world, Leo would have been only surprised because a rank 9 hero was enslaving an entire tribe that was protected by a level 5 supreme-ranked hero and level 6 supreme-ranked starlight cultivator.
However, Leo had already concluded that the Slave Marker in both worlds were the same, making him thoroughly shocked to know the strength of the Slave Marker.
'How can he be a rank 9 hero? How can that be?'
When he learned about the existence of the Slave Marker in the [Treasure Land], Leo had thought the Slave Marker to have at least level 1 supreme-ranked hero strength.
And when he learned the Slave Marker in the Beastnet was the same one as the human world, Leo increased his strength to higher than level 1 supreme rank because of his ability to enslave an entire tribe filled with people who were not only heroes but also cultivators.
Therefore, Leo found it a little hard to accept that the Slave Marker was only a rank 9 hero.