Interesting quote to say the least. As you stated, it is contrary to what has been stated here. My knowledge is limited, however, I have discussed this issue with 2 different individuals that build battery boxes. Both of these individuals informed me that their boxes were designed 'based on a generic impedance of 10 KOhms'. Both of these individuals also stated that I would need to correlate the actual roll-off values, as based on the input impedence of the recording device used.
Most of our recording devices (if not all) do not utilize the 'generic impedance of 10 KOhms'.
Like I stated above, this is only what I have been told by people I believed to be knowledgable about the subject.