It's a bit hard to define an antagonist in the South Park universe anyway. You do have some clear cut examples of antagonists, like Saddam Hussein and Tom Cruise for example, but asides from them and a few other examples, the line becomes a bit blurred; for example, Stan could potentially be considered an antagonist because of the role he plays in "Mystery of the Urinal Deuce", as well as characters like Bebe because of her actions in "The List", Craig for his encounters with the protagonists in "Quest For Ratings" and Sheila Broflovski for her role in the movie. However, those characters are also protagonists (for the most part) or simply neutral.
South Park Studios wiki is maintained by public editors and has nothing to do with South Park Studios writing staff. Just because their website hosts the wiki does not make their information any more correct than this wiki. I only trust the real sources as in The DVD/Blu-Ray/VHS credits, what the creators Matt and Trey say in commentaries or interviews, Imdb (which can be wrong sometimes) and interviews with the actors and other staff on the series. Since their is no proof it is actually April Stewart it could also be Mona Marshall, Elisa Gabrielli or Anna White. So come back with real proof.