Honestly, I don’t really care, and Kaufmann gets a few things wrong too. 
I don’t even start with science.
At the end of the day, if you reach fluency with 11k known words, good for you.
You might be interested to look at this old thread as well about his experience with German at 50k known words, and afterwards at 100k known words.
EDIT: in this thread there a bit of ratios calculations per book though.