I am always skeptical about any surveys done by by non-statisticians. I am even more skeptical about the interpretation of the findings done by such non-statisticians. Again, I am disappointed this time.
What kids would use Putonghua with their peers in Hong Kong? Except for a small number of Singaporeans and Taiwanese, I think the majority of such kids come from new immigrant families.
Their below average performance in reading comprehension is probably due to factors other than the mere fact that these kids talk to their peers in Putonghua. If we looked into the performance of these groups in other aspects, my guess is they would also come below average.
My comments are not meant to be discriminative. New immigrant families on average are less well off than the average family and therefore are less able to provide the resources, particularly parents' time, to the children.