Interesting read. As they noted, they assumed that the distribution of miners is representative of the three pools they surveyed, and this may or may not be true.
Those 3 pools represent close to 40% of the total hashrate which is a fact judging by the number of blocks they find, so I agree, it still is a matter of accuracy, but the fact is, the numbers in China are more likely to be more than shown in the study, simply because there are many large farms in China that mine solo, while most other non-chinese miners are mid-range operations who mine to pools, so if the study shows 70% at any given time, it's probably 80% or so.
and there is a decent chance it will drop by the maximum 25%.
Are you sure the maximum is 25%? I thought the new difficulty can't fall below 25% of the previous, since the current diff is 19.9T, the lowest it can go to would be 4.97T, but nothing stops it from going down 25% or 50%, correct me if I am wrong.