I think it demonstrates something stronger -- we have, as humans, already developed a game (Chicken) with very meaningful outcomes in which lower intelligence is beneficial, despite the fact that the humans in questions were not intending to select for low IQ and would not have seen a rock as a valid player.

If we are talking about Chicken we do not have to assume a rock (which has no preference), but simply a human with bad judgement, or slow reactions, or who is panicking.

So,

I'm not sure what that proves.

Well, 'proof' aside, it demonstrates that:

stupidity can be an advantage

Among other apparently maladaptive responses.