But it doesn't explain why, and what the mechanism behind that statement is. I configured my internal DNS servers on my home network to be root top level domain servers for a made-up .tld. Why wouldn't that work? Don't know, because the author does not explain the mechanism in detail.
As far as I can see for the past 15 years, having configured the internal DNS servers to be root top level domain name servers works like a charm, with no observed ill- or side-effects.