IMHO contexts are the modules in a modularized monolith. They encapsulate some functionality + schema and queries. They are like microservices within your app. And can be owned by different teams, when you're hiring a lot of engineers. Flattening this structure, wipes out all the modularity.

Yes, they can function as such but the problem here is still that the contexts might not represent your bounded contexts. Often, you’d have multiple Phoenix contexts in a single bounded context, so I’d be careful with trying to align these two