Retired English Teacher
I checked out the web page and it's way too oblique and nerdy. Most common is most popular and that means a simple number is attached somewhere.
In The Rocket Review Revolution, published by the New American Library, a book about passing the SAT, it lists on pages 112-3, the most common nouns, verbs, adjectives, adverbs and pronouns. Yet there are no numbers provided and no footnotes attached. How can we trust them if we can't verify?
I don't think it is that common- any attempt to establish such a list will be based on a sample, because it is impossible to analyse everything. Therefore, there will often be slight differences, especially as you move away from the top five or ten. These lists generally show quite a bit of agreement, as long as they use a sufficiently large sample size, but there is no absolute official list.