I don't really see why the author needs to justify it. New languages with new syntax come out all the time, some become popular some do not. Sometimes a new language comes out with a different syntax and becomes popular, creating a new norm.
But in any case Dhall does actually follow a norm rather closely, just a different norm than the grandparent is used to. Namely, ML and other functional languages/research. Which makes sense since Dhall is a product of that community.
Yeah, it deviates from the norm and the expectations of most people, most of who are not even developers - lots of BAs today configure apps via such files.
This is a valid criticism. There are already widely-used data formats; if you do something different it should be justified.