There actually is an accurate and justified explanation to why Trey and Matt made fun of Robert Redford & the Sundance Film Festival in this episode.
A mere couple years before South Park, before the Spirit of Christmas even, Trey and Matt had made a film in college called "Cannibal: The Musical"
After Trey had completed the film, he and Matt sent the film to be submitted at the Sundance film festival in Utah, along with their $50.00 submission fee. Not only did they not get picked for the festival, they never got a response of any kind from Sundance. And from the looks of it their $50.00 check never got reimbursed. What's more, according to Lloyd Kaufman, the creator of the Toxic Avenger and producer of Cannibal: The musical, Matt and Trey actually went to Sundance and created their own film festival opposing Sundance and Matt and Trey's festival featured only one film. and, you guessed it, it was "Cannibal: The Musical"
So, artistically speaking, Matt and Trey had every right to make fun of the Sundance film festival for not getting picked and for taking their money.