What’s interesting about this is that it used to be far more common for most technical people to be pretty well versed in basic calculus. A frightening number of software engineers don’t really understand calculus. But back in the 90s most engineers, even software folks who may likely have come from an EE background, would have had the basics down pretty well. DFW was writing for an audience with a potentially much higher mathematical literacy than today.

The probability that this “mistake” is intentional is related to how likely an informed reader would be to recognize this mistake.