A few decades? I think you misunderstand Aman777. He's the only creationist I know that views the universe as older than modern scientific conclusions (as far as I can remember, he believes it to be 21 billion years old). From my understanding, he views that humans were created and placed in a specific, small protective area separate from the planet Earth itself. Like how my bearded dragons are in tanks that keep them warm and unable to move around the rest of the house. As far as I can tell, the "prehistoric peoples" Aman talks about, from his perspective, have a separate origin from "true humans" and never experienced Noah's flood at all. Rather, they evolved normally or, perhaps, were guided into evolving as close to human as the natural process can get, but they would never reach "true human" intelligence via this process. The "protective tank" that the "true humans" lived in was dissolved as it was dunked into a lake, causing the inhabitants to experience the flood. This is also why this was the first time rain was experienced in this tank, as holes began to form as it dissolved the barriers, referred to as "firmament" and the water flooded into the submerged "tank". Once this "tank" fully dissolved and the ark landed, Noah and his family found no other humans left. As a result, when it came time for Noah's grandsons to want to start families, they had to settle for Neanderthals and other "prehistoric peoples" that were not as smart as them, but were physically compatible for breeding. It is through this hybridization that modern humans get their intelligence, as well as genetic ties to organisms such as chimpanzees. Through independent genetic lines.
I think this summarizes Aman777's views, though he is free to correct me if I got parts incorrect.