Noah wasn't underestimating the trial. Some of the features described by Don worried him greatly, but he couldn't give up on that chance to prove his power and seize the best reward of the second layer.
Don watched as Noah dug a cave near the edge of the landmass and secluded himself. He had no words to express how reckless Noah's behavior was, but he had already explained everything he could.
Noah wasn't even part of the Legion, so Don eventually gave up trying to make him change his mind. If Noah wanted to risk his life and waste time over an impossible trial, he was free to do so.
Don soon left, and Noah remained alone in that distant part of the separate dimension. The light radiated by the Divine Deduction technique shone on his mental sea and improved his mental capabilities as he reviewed what Don had explained.