I'm not sure what speculation in the media Idsinga was referring to. If you mean he was referring to KD, I'm also not sure that he has actually speculated about a family member. Admittedly I haven't read every word in KD's book, but I haven't seen him speculate. Perhaps alluded to, might be a better word. Unfortunately I can think of a reason why he might think it damaging if media speculated about someone AND said person was also being secretly considered a suspect. Further, I am reading from the quoted text that it was the family who said it was damaging to read..., and Idsinga was relaying the family's message.
From what I have read to date, it seems (to me at least) like KD is providing commentary from interviews he did with various people who were knowledgeable or close to the couple and situations. I may have forgotten since it's been awhile, but I don't recall him particularly expressing his opinion other than perhaps his own thought processes at certain times.
I can't say if I've ever heard police thank a family for their support, however in this case, since it was so heavily broadcast that the family were upset with several police actions, they got their own ME, they hired their own detectives, there was political involvement, and at time of thanking, the family had cut their investigative team loose and seemed to be dealing directly with TPS, I can only imagine the relief TPS may have been feeling at that time, all things considered.
As well, I wouldn't expect TPS to divulge their investigation or their tactics. In my mind, nothing Idsinga is saying translates into each of the family members having been cleared. Jmo.