I don't think the distinction of working with people vs working with things is clear enough to say wheter my job means I am doing the one or the other. So how could the participants in the study do the same? I'm assuming there are lots of occupations in this grey zone and even situations where in one company the same occupation is considered working with people while in another company it would be considered working with things.
I can see multiple issues the study runs into which would be interesting to see how/if it answers. A) Does it measure peoples perception of wheter they work with people or thing? B) Do the authors make their own interpretation of which occupations lands in which category? How do they then eliminate their own biases in what working with that occupation means? C) Did the authors observe the participants and make a judgement call based on their day-to-day activity? This would likely be the most accurate, but I can't imagine they did this because of the sheer cost of such an experiment.