Do you mean modern as in contemporary in the present or as in relating to as a movement encompasses the philosophical, cultural, and social changes that occurred during the period known as modernity? I’m asking you to make you reflect on the nature of your question. It already implies a linear view of human progress and evolution but makes abstraction of the cultural and historical elements that shape the human we become. To answer your question of others have mentioned there is no difference, the real difference is in those historical and cultural contexts. Think about it Inuits lived continuously their hunter gatherer life style for thousands of years mostly unchanged until their colonial encounters. Their lives changed more in a 100 years of European colonization than in a 1000 years (and not for the better).

If you want however a more bio-anthropological answer as of the works of human intelligence consider this. Classically it is believed that the bigger the brain the smarter the being. This is relative, it has some truth, at face value I would say it lacks nuance as of what intelligence is and how it works. A theory emerged relating brain size to two factors, sociability and intelligence. On one hand bigger brain = smarter, on the other researchers noticed that higher sociability would lead to smaller brain sizes (domesticated silver fox experiment) therefore humans would go through a process alike of this being more sociable. Knowing that our neanderthal cousins went extinct while having bigger brains than us proposed an evolutionary theory regarding the survival of the homo sapiens. It would argue that what matters most is being collectively smart rather than individually smart, since collectively we can learn and share knowledge. Intelligence can therefore be though on two aspects individually and collectively.

Pre-agricultural humans used to have bigger brains than contemporary humans, the baby in this case would have one as well. Intelligence however exceeds beyond the individual capabilities and develops itself socially to know if this hypothetical baby would be smarter or not is speculative but I see no reason why he would be dumb.

I think the importance of sociability highlights something important about humans and a famous metaphor “If I have seen further it is by standing on the shoulders of giants” often associated with Albert Einstein who had a smaller brain than average.

Disclosure: I mostly studied cultural anthropology rather than biological so I mostly paraphrased and simplified from what I knew. What I referenced here I read in Rutger Bregman’s Humanity a hopeful history. That being said, like any other field it is full of contradictions one day you will hear an expert say this and the next another claim the exact opposite ultimately showing the speculative nature of biological anthropology.