After reading some of the novel, I wanted to skim through the Manga to see how they drew certain scenes, and they've added so much. This whole bit about being eaten, and the vision that came because of it didn't happen in the novel. The Earth Dragon was sleeping in the bottom of the pit, and woke up shortly after Rentt noticed it. It was enough of a shock to kill any ambition he had to test his strength and he goes home straight after.
It seems the author decided to be a lot less subtle about certain things, and also introduce plot hooks much earlier. I'd guess the novel's pacing didn't make for a suitable 1:1 adaptation, so this is probably their way of adding some filler panels while also increasing the pace of the story.