Not everything can be abstracted away. In some cases, it's simply not possible, and in others, it might not be convenient. Let's start with the convenience aspect.
In this example, there is an adapter of the web framework of choice to a clean API to be presented to the application. In a more complex scenario, such a change might not be possible. Even with this abstraction, parts of the library were still visible to the application. Adapting an entire framework might not only be hard but also not possible in some cases. It's not entirely a problem to be completely isolated from the web framework because, sooner or later, we will need some of its features or technical details.
The important takeaway here is not the adapter, but ...
