What exactly in the UNIX philosophy says configs should be editable via text files? It specifically talks about CLI tools using plaintext for their I/O to allow piping commands - not about configuration.

It's called the "Rule of Textuality", a component of which is: "Store data in flat text files." This principle recognizes that text files are human-readable, easily editable with any text editor, version-controllable, and can be processed by standard UNIX tools.