Yes, Christianity is a cult but not really in the tabloid, cable news meaning that you are used to. “Cult” is not inherently negative or out of the mainstream. It simply means a system of religious worship. It has become more negative in recent decades in popular culture by being associated with some “unusual” religious groups.
I don't mean it as a slur, really but Christianity started as a cult, though it can no longer be construed as one for the most part - aside for certain sects (though I'm unsure about Catholocism and their fealty to the Pope above almost anything... it seems almost antithetical to pure New Testiment teaching, but I'm non religious and wasn't raised Catholic, so much of it is a mystery to me). This religious group they're describing in the article, however seems very out of bounds for the term "cult" and they seem to use the term in merely a pejorative sense.
As far as the legal definitions of cults go (under US law), well there it gets "squishy" and kind of comes down to a popularity contest, almost. "Are you of a certain age", "Are you of a certain size", "Have you a certain amount of influence", type of thing.
