It's good that testing is covered as part of your teaching strategy, because as you said, in any real world environment that experience will come in handy.
But the audience reading and sharing links on Hacker News are likely to have a deeper body of knowledge in this area than pure beginners. People posting tutorials (for free) may be targeting this more experienced group of people, who may take it for granted that a beginner's tutorial will not cover everything necessary to go from a prototype to production-ready software.
I also feel that as people progress through tutorials, they will organically come up with questions that the tutorial has not covered at all, like "how can I test this to make sure it's bug free and can withstand lots of concurrent logins?"