I think you’re correct. The story on our species is known to degree that it’s unreasonable that an advanced agricultural civilization existed. I think it’s more likely that far further back, a dinosaur species achieved a more advanced state than we think possible today. Not quite agricultural due to a lack of tool use, but as far as you can go without the opposable thumb. Advanced social structures and nomadic. It’s pointless speculation, but I do think it’s more likely given the timespan available.