“and all but the difference between the postulated little objects and big objects is not enough. “
I think the article says that lots of little objects have the same influence as one big object.
Yes that is what they are saying. First the orbital periods of objects beyond Neptune are going to be on the order of hundreds to thousands of years. Second randomness would put those objects roughly evenly around the sun at least at the beginning and they would resemble a very sparse asteroid belt. Thus the gravitational pull of the little objects would be distributed over large distances.
The article was trying to say that somehow the little objects would cluster together in a smaller area where they would exert a combined force. I think if one were trying to infer such a thing happening one should look at similar systems to see how they behave. I suggest planetary ring systems and the asteroid belt.