The TL:DR version of this is quite simple. Trumps poll numbers actually do not move all that much. They change because the ratio of democrats:republicans change in every sample.

In effect, the pollsters are creating the illusion of movement in opinion where there is actually very little. Note the author does not say this is intentional, just a statistical fact.

This jives very strongly with 2016 where the most accurate polls were the polls that had the uniform samples (in fact THE most accurate poll polled the same set of people over and over again).