Remember that the argument is stating that one way you can definitively tell how quickly a new ides is taking hold among the population is to watch how fast that word or words expressing that particular idea, i.e. the word must still hold the same meaning when it is transferred to common usage, pass into common usage.
Think about it this way, if a word's meaning changes as it crosses into common usage, then we can no longer say that the idea behind the word is spreading or raking hold among the population. For instance, we cannot say that the fact that young people might say "cool" to something they find pleasing or interesting means that the word "cool," meaning cold in temperature, has taken hold among the population, since the original word had a different meaning.
Thus, specifying that a word's meaning does not change as it passes into common usage helps strengthen the method that is being advocated for as a good tool in determining whether a new idea has taken hold of a population.
Hope that clears things up! Please let us know if you have any other questions.
