I think he was serious. The totality of existence is a human construct. Suppose I say all Greeks are mortal. What does "all Greeks" mean ? Is it a thing ? At another stage in the debate, Russell says that every man has a mother, but that doesn't mean the human race has a mother. It's a completely different concept. Similarly, if we agree that everything in the Universe has a cause, it doesn't mean the whole has a cause. They are different logical realms.