I spent a while on this one deciding whether to go with what I understood to be the correct answer based on the instructions, or to aim for replicating the example results. I've decided to share my answer with the former option:
Like many people, I initially struggled to understand the meaning of the 'Positive Feedback Count' field. I checked the source page which Christine kindly provided, which defined this as the number of times an individual piece of feedback has been rated as useful by other users. With that in mind, it made sense to count each piece of feedback with a 'Positive Feedback Count' of 1 or more as a positive feedback review, rather than just filtering to reviews with 10 or more in this field.
The other decision I had to make was whether to include ratings from reviews which had not received positive feedback. I concluded that it made sense to include these, and to consider the 'at least 10 positive feedback reviews' as a test for whether a particular item has been reviewed enough times to have a fair assessment, and that all reviews for these items should be included in the average.