• The description is a bit vague, but the article seems to describe a model of a stack of resolvers, each resolver asking the next one in turn, the last resolver getting an answer, and the answer being then passed down the chain again. But this is not how it works. For a normal DNS query, there is ever only one resolver involved. This one resolver is the single thing asking many authoritative servers the same question, each time getting names of servers who might know the answer, and ultimately getting an actual answer. There is no stack of resolvers.
• The article completely conflates the process of a cached record expiring due to TTL expiration, with the process of an updated zone with a new serial number being transferred to slave servers. These two things have virtually nothing in common.
I also found the illustrations too detailed (i.e. confusing for non-experts). There are a lot of options to "dig" which could have been used to cut down on the extraneous output when illustrating a particular point.
The zone file was especially confusing, in that none of it was explained, and the nature of the DNS data model would have been much better illustrated with an object diagram of some kind.