The article is written for those who want to apply DDD/onion architecture to Python apps using Pydantic. Those concepts explain the motivation and the article assumes the reading knows about them. As others are writing, it may not be worth it to apply this to simple apps, but as an app grows in complexity it will help make it more extensible, maintainable, etc.

I'm not a Python expert, but looking into it briefly it seems like Pydantic's role is at application boundaries for bringing validation/typing to external data sources. If you are not working with external data, there is no reason to use it. So, if you separate out a domain layer, it brings no benefit there. Creating a domain layer where you handle business logic separately from how you interact with external data means those layers can evolve independently. An API could change and you only need to update your API models/mapping.