Yes, it's totally a hoax. They went and picketed with signs and handed out fliers. It's just a "social experiment" they say, showing how gullible people are, and so they can make fun of the hate mail they get. It's generally over the top but "believable" in that it's very much in the style of folks who want to condemn pop culture as being satanic.
I am almost 100% I know which one you're referring to, and I could probably find it again if I wanted to. They play it like it's real, but it's not.
But don't feel bad, some people think Landover Baptist Church
or The Onion
are real too. Quite frequently in fact...