This possibility doesn’t undermine the argument, because it concedes that “crying has a tendency to reduce emotional stress.” We want to point out why crying might not reduce emotional stress; that’s how we point out why the argument is flawed.

The stimulus doesn’t present any condition that’s required to produce something else. We’re never told that feeling stress is required in order for the body to produce the hormones. Maybe the hormones can be produced during other times, too, beyond just times of stress.

This possibility doesn’t undermine the argument, because it concedes that “one phenomenon causally contributes to a second.” We don’t want to concede a causal relationship. The flaw must related to why crying does not reduce stress, or why hormones do not cause stress.

The premises don’t present any factors that “are jointly responsible for causing” anything. We don’t know what causes stress, and we don’t know what causes the hormones to be produced. We have no basis to identify anything as jointly responsible for causing something.

The argument assumes that because the hormones are present whenever emotional stress occurs, the hormones are a cause of the emotional stress. This assumption underlies the author’s belief that getting rid of the hormones will help reduce stress.