The article doesn't make that claim. For example, the service n7 is used by multiple other nodes, namely n3 and n4. There is no cycle there, so it's okay.

Treating N4 as a service is fair. I think the article was leaning more toward that idea of N4 being a database, which is a legit bad idea with microservices (if fact defeating the point entirely). My takeaway is that if you're going to have a service that many other services depend on, you can do it but you need to be highly away of that brittleness. Your N4 service needs to be bulletproof. Netflix ran into this exact issue with their distributed cache.