The point is to demonstrate the obvious repercussions of truly trying on the belief "free will does not exist/not a meaningful thing" on something even so commonplace as an online conversation. If there is no free will, there is no point in having a conversation. People can't have opinions or be convinced, they can't decide, they have no agency. The whole language of rational conversation falls down like a house of cards.
It is an argument ad absurdum that is meant to demonstrate how naive it is to try to prove that free will does not exist (naive and disingenuine: "prove to <someone>" already implies <someone> has a free will to ultimately agree or disagree with the presented proof).