I think the testing model is flawed though since users are give a task with no opportunities for distraction or to make choices outside of what they're supposed to be doing. In this scenario I believe that they're right, multiple monitors are faster.
I still don't believe that they're more productive for most people when they're given free reign over what they choose to focus on. As I've been forced back to macOS while iPadOS is too buggy for some of my tasks I've seen that it takes me longer to get my number of productive hours in the day done.
I want to see a study that looks at what real world people do with their big screens vs multiple screens vs iOS vs other tablet operating systems. No contrived tasks in a lab setting, but what do people actually do when given freedom to do what they want. How does screen size and operating system windowing model affect that?
If you've heard of this study, I want to know about it.