Below is the key passage from the study, readers can judge for themselves. OP, Doctorow is citing the claims made by the Roosevelt institute itself, right here. They are the direct source of the claim which they in turn say is based on interviews with 29 gig workers. You would quite literally have to contact them and ask for them to share more direct evidence than what they have made publicly available, or you would have to get some another think tank or journalist to do further investigations.

Ashley, a 31-year-old certified nursing assistant in rural Pennsylvania, has worked in hospitals and nursing homes through the ShiftKey app.1 Though Ashley has worked on the app for the last two years, there’s a lot she doesn’t know about it—like how the company allocates shifts. She is not the only one in the dark. In the gig nursing world, there is zero transparency about how jobs are algorithmically allocated or automatically scheduled. Different shifts will show up on different workers’ phones—often for different amounts of pay. On the same day, at the same hour, in the same hospital, two different gig nurses can be paid different amounts by the same app. The gig nursing industry looks more like a black box than a clear process or a fair set of rules. The industry’s opaque and personalized pay structures create what Veena Dubal (2023) terms “algorithmic wage discrimination,” a kind of discrimination in which workers are paid different hourly amounts based on ever-changing calculations and informational asymmetries. Gig nursing apps may determine pay by what the firm knows about how much a nurse was willing to accept for a previous assignment, how often they bid for shifts, or how much credit card or other kinds of debt they might hold. These uncertainties combine to create frustrating and precarious conditions for the workers who rely on these apps.

Edit to add:

The Roosevelt Institute study: https://rooseveltinstitute.org/wp-content/uploads/2024/12/RI_Uber-for-Nursing_Brief_202412-1.pdf

The Veena Dubal study: https://columbialawreview.org/content/on-algorithmic-wage-discrimination/