This is a nice theory, but what is the proof? Were people just as smart 30,000 years ago? What about 300,000 or 3 million years ago? How do you set the time scale? When was the boundary when people became smart?
We don't know exact dates of course, but looking at the works of mathematics and philosophy that have survived, we know for sure that humans were capable of abstract thought at least as far back as 4-5000 years ago (for example, during the construction of the Great Pyramid of Giza), and very likely even 12-13000 years ago (during the construction of the megaliths at Gobekli Tepe, for example). Furthermore, we can assume that some time must have passed between the first threads of abstract thought and any achievements due to it that would stand the test of time, though I don't think we can put any hard estimates on this time.
We can look at the genetic changes and timeframe of them, and how certain mutations have spread - novel changes in our species are not that frequent on the historical timescale. People 3000 years ago are the same with respect to their biology, so they have to be "just as smart"; hominids 300,000 years ago are different in some aspects so for them we can't really know how "smart" they were, perhaps they were as smart, perhaps they were not.