People using smartphones while driving are absolutely distracted. Whether they think they are capable of managing that distraction (and how well that perception actually maps to reality), the distraction is understood and explicit.

In contrast, one of the challenges with FSD is that the driver often acts as though there's no distraction at all, and that their inattention to the road (apropos of detecting 'attentiveness') is a non-issue "because the car is driving and paying attention".

That's what this study is about - that markers that Tesla and other manufacturers are using to measure attentiveness may not be particularly accurate.