Souta told the layout of the Dark Light of Eternal to everyone first. He explained to them that this was the only thing that he knew in this dungeon. This was enough for everyone to know some of the dangers hidden inside this high-level dungeon.
In case they ask how did he know the layout of the dungeon, Souta said that he scouted this dungeon yesterday after he cleared the mid-level dungeons with Yuko.
He only scouted it which means that he didn't know the ins and outs of the dungeon. Even though Souta knew this dungeon in the game, there's still a possibility that it was different in this world. He encountered a lot of things different from the game so he was prepared if something happened.
"So I'm going to ask you, captain. What are our chances in clearing this high-level dungeon?" A man with mohawk style hair asked him. He was Ginvi, the earth attribute mage of the Lanny group.