The interviewee addresses that:

> All the stuff where incentives destroy incentive, motivation are things that you like to do in the first place...

> These are, frankly, inner-city schools with low achievement. We were trying to foster a love of learning, getting them to try it and see that they can excel at it, not the other way around. We also did all the measures of intrinsic motivation, we found no effect on it whatsoever. So we didn’t foster it, and we also didn’t hurt it.

Considering the baseline "incentive"/"justification" for going to school in the first place is "because I said so", the incentives here are certainly additive because x+0=x.

This would be a great deal more impressive if most contributors to the largest and most subsection OSS projects weren’t paid.