They are both serialization formats that are supposed to be able to represent the same thing. Converting between these 2 formats is used in the article as a way to highlight yaml's parsing quirks (since JSON only has a single way to represent the false boolean value, it makes it clear that the no value in yaml is interpreted as a boolean false and not as the "no" string)
Anyway, I disagree with your point about YAML and JSON not being interchangeable

It's often a good idea to make the code itself very explicit through verbose function and variable names, rather than writing comments that could lead to inconsistencies between code and comments (by not updating the comments at the same time as the code) (see "Fallacious Comments" from the catalog)