I don't think he was joking.
Hubbard wrote that he was, essentially, assuming the role of the anti-Christ, rather than being the actual anti-Christ, since while, according to Hubbard, Jesus existed, he was not the Christ. Indeed the idea was that there was no Christ, and the Christ notion was the result of an "implant."
Crowley has a similar situation. He regarded Jesus as just a man. Crowley was not a Satanist in the religious sense as he also did not recognize the divinity of Jesus, but he was a Satanist (or very much like one) in the philosophical sense.