I am a fan of Pragmatic Dave and I share some of this ideas. In this specific case however, it is my personal opinion that a formatter file does not hurt because this way, your team can have a custom formatter file in a public Repo and use it for all projects.
There is also the notion of mixing concerns.
mix.exs files usually are concerned with dependencies (compile time or run time), and apps that add QoL to development (such as credo, for example). If you add formatting options to mix.exs, then should you not also add the configuration needed for all other mix deps (should you not also add credo configurations to the file since credo also has some formatting capabilities)? The line gets very thin very quickly.
I defend that one should have small projects, but you also need to understand that PragDave’s approach deviates a little bit from the community standards. In the end you really have to consider if the usage of the community standard satisfies the costs it implies.
If you want to help newbies, then my recommendation is to follow community standards because this way they will be able to benefit from the help of the entire community when they finally hit a wall.
If you opt for a custom approach to projects, then the community will have a harder time to help you and your team because your solution deviates from the standard everyone is used to working with.
When did this happen? Can you share?