None of these "researchers" would ever wonder why a person would spend time playing in a string quartet when they could, economically, be making money behind the counter at Arby's. The idea that doing the work is rewarding never occurs, apparently.
Even by the second page of the paper it's very clear the study is designed to read off intrinsic motivations, both value-laden and pure enjoyment:
This framework also accounts for potential interaction effects between intrinsic and extrinsic types of motivation. For the intrinsic and extrinsic motivation subscales, 36 items are included in our questionnaire. Each subscale (i.e., enjoyment based intrinsic motivation, self-reinforcement, obligation-based motivation, integrated regulation, identification, introjection-based regulation, external regulation) consists of four to eight items...
The real question (which the authors are silent on and the journalist helpfully mischaracterizes) is whether these researchers' results generalize to non-R-ecosystem OSS development.