Of course it's all stored in the filesystem, so there isn't anything that isn't technically a file.

But, for instance, while as before you'd have a simple log file you could access directly, now you have a binary blob that is inaccessible without systemD itself.

The are other numerous better exceptions that are better examples than systemD. DBus, for instance. InfernoOS is an interesting example of taking the "everything is a file" to heart.