Those are called Public Suffixes or effective TLDs (eTLDs): https://en.wikipedia.org/wiki/Public_Suffix_List

And you're entirely correct that author should've referred to such list.

I think the problem is that the original source needs to use that list as well. Just looked though .nz and they list several sites ( govt.nz , school.nz, gen.nz ) that don't exist since all the domains are one level below.

They even list.govt.nz as the top site. In fact that doesn't exist (although www.govt.nz does since it is a a kinda government portal )

I see they list an old employer of mine who got bought 15 years ago and whose website has been redirecting for 10 years.