Like when two people are in an argument and the one who is losing says: "Well I don't care what you say. You're a Towel!"
If you're a South Park fan and you remember a character called "Towelie," then you don't need this explanation. If you're clueless or need a brief refresher course, here's a short video.
Towelie was a joke character created for South Park to make fun of how much the show was being commercialized at the time. He showed up around episode 144 to randomly remind kids to pack extra towels. Whenever caught in an argument and challenged on the fact that he was in fact a towel instead of a human, Towelie would reply with "You're a towel!" Only to be followed by, "No, you really are a towel." Then he would say: "No -- you're a towel!"
In this toxic political climate where people are quick to hurl insults or gaslight instead of civilly and respectfully debating issues with fellow human beings, this Towelie quote is particularly appropriate.