As it turns out, the abstract of the (not yet peer reviewed) paper the Verge article is reporting on doesn't specify whether self-reporting was leading to over-estimates or under-estimates of screen time either. Most of the authors are academics working out of university psychology departments in the UK, don't they think that's an important part of their results to report? https://psyarxiv.com/6fjr7
I wonder who is funding this research, and brokering the agreement with Apple to access their data? Curiously, one of the authors is one Brittany I. Davidson from the "IDO Division" of the School of Management at Bath Uni. A quick web search turns up the Information, Decisions, and Operations Division, a group which appears to be fairly ... let's say industry-friendly: https://www.bath.ac.uk/research-groups/information-decisions-and-operations-division/
So in summary, a collaboration between a major manufacturer of mobile devices and provider of internet services, and an industry-friendly research group, has produced a paper - not yet peer-reviewed - whose only conclusion is that we should all be more skeptical of the large body of research indicating that mobile devices and internet services could be causing people some harm. So why is Rachel Becker, "science reporter" at #Verge, so keen to publish on this? https://www.theverge.com/authors/rachel-becker