Why would the speaker say "then" in the end?

Native speaker’s answer
Rebecca
Here, "then" is an adverb that means "in that case" or "therefore." So he's saying since that's the case - since the commenter challenged them to do this - they should go. Ex: Then if you're free, why don't you hang out with me this weekend? Ex: A: I'm not hungry. B: Let's just go home then.