Did you even read the first paragraph?
"How is it that a survey of only 1,000 people can reach this level of accuracy? You must first assume that the survey respondents have been sampled at random from the population, meaning that people are selected one at a time, with all persons in the U.S. being equally likely to be picked at each point. For most polls, this is approximated by calling phone numbers generated randomly by computer."
In other words, self selected participants are not selected randomly. If they, as pointed out above, all pollsters would have to do is do online polling with no filters. To most people with even a rudimentary understanding of probability, that obviously doesn't work.