For error-related negativity (ERN), "negative" refers to the direction of voltage change, not the person's attitude.
Since ERN amplitude is associated with better learning, it might be the other way around, that people in flow are producing stronger ERN's and correcting their mistakes faster.
I suspect that Moser/Dweck would find that programmers in a state of flow encountering a compile/runtime exception would exhibit significantly lower amplitude signal associated with immediate negative response.