If the idea is true, does this apply to the Greeks of Ancient Greece as well? Ancient Greece seemed to have been rife with geniuses unmatched with anyone of modern days. What about the general masses of Ancient Greece? Were they as intellectual as the high philosophers, such as Aristotle for example?
To be fair, reading works over a thousand years old, I seem to find it likely that those authors would still be more intelligent than the average of today's modern age.
I am not well versed in the modern history of perceptions of the past, but I would venture a guess that the idea derives from the Enlightenment concept of progress. With the notion that we are advancing towards ever greater knowledge and understanding of the world around us comes the corollary that people in the past must have known and understood much less. In the Early Modern period, of course, the convenient foil for contemporary "enlightened" thought was the supposed barbarism of the Dark Ages, in which - so people thought - religious zealotry and irrational superstition ruled supreme, and science and technology were stuck at a primitive level.
As for the Ancient Greeks, we must not idolize them. Bear in mind that the written sources we have for the period are almost without exception written by wealthy, educated, upper-class men (with the few exceptions being wealthy, educated, upper-class women). These were very much in the minority, and the philosophically inclined among them were a minority within that minority. As far as we can tell, the lion's share of the leisure classes in Ancient Greece preferred to spend their time hunting and drinking rather than advancing their knowledge of the natural world. Philosophers spread their ideas among the few who took an interest, while most people probably regarded them as tiresome pedants. If you're wondering whether the average Greek was intellectual, I would recommend reading Aristophanes' Clouds - a comedy entirely devoted to ridiculing Sokrates as a hobo and a charlatan whose only goal in life was to corrupt the values of upstanding citizens.
In all likelihood, people in ancient times were just as smart on average as we are, in the sense that their capacity for rational thought was the same. However, they had almost no access to education or information. Only a few had the means to rise above what could be learned from oral tradition, practical experience and folk wisdom, and even fewer saw the point of doing so. Those who did may still impress us with their insights, but even the wisest Greek knew infinitely less than we could learn from simply browsing Wikipedia.