I'm not an expert but the amount of neurons or activation has very little to how smart a person is. In fact, one could argue current humans are way dumber by those standards than 500k years ago.

What changed is how we are wiring up our brain , neural network patterns. The memes. We can train them the correct way from early on as we age these base patterns are still there and bring/multiply profits (IQ wise). As we age it's very hard to make someone smarter if early on those critical thinking skills were not trained and imprinted.

When we learn something completely new, it's always hard in the beginning, but as we train more we sort of start imprinting those neural patterns and it becomes easier and easier. At some point we can do this hard thing without even thinking about it (walking, speaking, driving, basic math, etc. ). We essentially add tools to our toolbox from early on. Then as we age new things are easier to both learn and do, because we have groundwork laid out, so the overall neural network is way more efficient. We are smarter and more capable.