The coaching could easily go on via personal messages or text messages.
At least if people make their opinions known in the comment sections then it makes it harder for them to vote for the person they are coaching as everyone can see that right from the start this person has been biased to his or her personal opinion on the subject.
Also it would be good for a debater to get a whiff of what the voters are seeing, as having the opportunity to also debate the voters and prove them wrong would be an advantage to the debater, not a disadvantage.
But it really does not matter where a debater gets his or her information from. Whether it be from Wikipedia. Newspaper. Book. Bible. Youtube. Another user. What difference does it make?