Am I right in that this part of the answer is no longer in the Core-reading, as is the relevant section of the notes? Page 12 of Chapter 11, "Properties of Rank Correlation"
The 2015 syllabus states: "4.2.2 Describe different measures of correlation and discuss the relative merits of each for modelling purposes." Kendall's tau is discussed in required reading in Sweeting (9.5) So, still examinable I would say.
I think that solution (and the corresponding ActEd notes) are based on the old McNeil text - but similar concepts are explored in Sweeting.