The answer was in the question I think. I believe that the reason that I couldn't call the method that I created was that I didn't have a leading slash in my path
argument of the method call DBus message. As in:
/com/example/whatever
correct
com/example/whatever
incorrect
The guide I referenced in the question has the minimum requirements to do it. I believe that the reason that I couldn't see the method in D-feet is because it relies on introspection method calls, which I did not implement.