This is not a bad overview of DNS from a theoretical perspective. It’s also pretty well written and has good examples and figures.

What I think is missing is a bit more of the “in practice” side. If the author was surprised about TTL values, I doubt they have much experience with some of the other pitfalls, so I’m not surprised (not a knock on the author). But there is a reason why the phrase “It’s always DNS” exists.

As an example, it could be helpful to mention that ISP DNS resolvers (or any caching resolver in the path) could decide to ignore the TTL. In this case, your 360 sec TTL might not get updated for an hour or a day or longer. This can be infuriating to troubleshoot.

A section on troubleshooting might also be beneficial. But this mainly consists of checking results from different resolvers in your path - does it work with a local resolver? Your ISPs DNS? The authoritative server?