It's an example, used to demonstrate a concept, rather than a concept in and of itself.
The concept is that you don't make an architectural decision until the last possible momentlast responsible moment, but no later. But, in reality, you often do have a decision on the database you're going to use very early on. That might not be perfect, but it's a fact.
Once that decision is made, you don't actively avoid it. Storing something in an existing database is often as easy as storing it in a flat file.
But changing from MySql on Linux to SQL Server on Windows might not be as simple as changing from a flat file anywhere to SQL Server on Windows. That's the real point. So, while there's doubt as to the final solution, take the simplest possible option, accounting for change. Once a decision is made, stick to it.