Apparently, law enforcement did. Whether it was serious or not is irrelevant, if a threat was posted LE is not going to ignore it. I think it's exceedingly stupid to post on the Internet in this fashion, because you are essentially begging someone to investigate you if something should happen to the person who was allegedly threatened.
If the same make believe threat was posted about some form of terrorism as a joke it would get investigated. Do you think LE is paid to have a sense of humor or to investigate what appears to be a posted threat?
As the saying goes, what the Internet does is make smart people smarter and dumb people louder. And sometimes it is not so good to be so loud, because the LE will hear you.