I have to say, this doesn't sound like a "multi-tasking" study. It sounds like just another memory study (a multi-memory study), but this time it's with a larger amount of variables. And they say multi-taskers are in a constant state of stress....well, maybe sometimes, but I have to say that for most of my day I'm relatively calm and have a system for all the assignments and duties I have to perform.
I think we can all agree, as SMs we do have to remember a large amount of facts and information over the course of a day (let's not even talk about over the course of a show or season). But that's why we have note-pads, lists, and forms of organization. If anything, I feel like SMs are the exception to this "Lousy at everything" conclusion. We find ways to get around the inefficiencies of the human condition and make things work. Of course we don't remember every color of people's clothes and what they brought for lunch and who called us when. But we can build a prop and fix a costume, all while monitoring a show before curtain and giving places and working with a egotistical member of your crew.
And lets think about how other jobs also do the same things as us. How could you possibly run a company, or a store, or an office, or fill any managerial position without the ability to multi-task.
Personally, I have to declare shenanigans on this study just from what I've read.