I think part of the joke is that the high speed trading does not meaningfully lead to decreases in infant mortality, but the high speed trader is doing a lot of mental gymnastics to try to claim it does.

They are, the joke is that the guy is trying to weasel his way into heaven by claiming that the shitty things he did on earth were a net good via bad leaps of logic. St. Peter knows the guy is full of shit, but lets him think he’s getting into heaven for a bit.