Inside jokes, or humor of any kind, have no place on HN, Ben. May you be stripped of all your imaginary internet fun points, so that you learn not to be such a light-hearted and well-adjusted individual.
Well, should you take a cultural relativist POV then there's nothing "true" about Kantian ethics—it's a choice of values. It's interesting to see 21mo behavior selecting for those values.
If you take the deontological view then this is still a fairly complex behavior to execute. To consider whether this sort of judgement is one that should be fairly placed on the 21mo if it did the same is a challenging mental task.
So, perhaps babies are predisposed to heuristics which favor the categorical imperative. That'd be interesting and certainly worth surprise.