There are plenty of sentences that have a noun, a verb, and a subject without having an agent - anything in passive voice or any unaccusative will do the trick. I suspect the argument would be even better worded using semantic roles rather than syntactic categories, eg: "Causality exists when there is an event with an agent". This isn't a very interesting thing to say though, because "agent" is a casual semantic role and so relies on causality existing by definition. You literally cannot have an event with an agent unless there is causality.