For the same reason so many people believe it, it's a fairly reasonable assumption before you really look into it, Youtuber loses litigious court case and then commits suicide. It's not hard to make assumptions and connect the dots even if it turns out they weren't actually there, but Karl is someone who probably read the actual suicide note so it is dubious. He probably just didn't think it was a big deal to claim it, based on the damage it caused Billy it honestly wasn't, but it was still untrue. He's lack of any remorse, his related Billy content and lack of a proper apology once corrected convinced the judge that that made the claims with malice for Billy in his mind, rather than it just being a careless and incorrect thing to say.