I think you are reading too much into it. They never said that there would only ever be one PostgreSQL db with everything. As you point out there are advantages to separating certain functions into separate dbs, but for most businesses that go that route, it makes sense to use multiple instances of the same core tool.
Also, wanting to have a single authoritative view of inventory and order data is NOT a big ball of mud. Nor is a single big db with everything. The bid ball of mud generally starts with the link editor and developers who think that the only way to integrate functionality is to link it all into one big binary. For all we know they are planning to use RabbitMQ as their main integration tool, and build lots of small single purpose apps that work together like an orchestra.
Also, wanting to have a single authoritative view of inventory and order data is NOT a big ball of mud. Nor is a single big db with everything. The bid ball of mud generally starts with the link editor and developers who think that the only way to integrate functionality is to link it all into one big binary. For all we know they are planning to use RabbitMQ as their main integration tool, and build lots of small single purpose apps that work together like an orchestra.