fly.io employee here, it's basically an adaptor from the Kubernetes world of YAML to the fly.io world of Machines. How could we have framed it better so that it was more clear?
In my opinion, the concept is not trivial to grasp by oneself (meaning: is not trivial to drive into that conclusion even after reading the article). So, being explicit first, and guide the reader through that concept along the article would have made a big difference.