The point is, the Arduino IDE compiles and links all the files it can, which finds in the working directory. Therefore, you cannot have anything it can compile in there unless it is part of the project.
So by inference, every source file that is in there as part of the distribution is necessary.
Alas…I had not idea what the config.ino file was for so I didn’t bother to download it. I have never done a multifile sketch so this was new to me. But thanks to you guys, I understand it now!!