Nope, you are right. And I read that in IG too. -ve OPK does not mean that O has already occured. The LH surge goes down just before you ovulate. Taking it twice a day helped me to know if it was in the morning or night. I def could not get the hr..lol.
http://www.fertilityfriend.com/home/319451
If you see the first chart which is the BFP one, we DTD in the PM, on CD 15. I did that after OPK went -ve. I did take a chance there on the fear on hitting O+12. But OPK was +ve in the morning, so I knew BD at night put me very close to O. CM was actually teh higest on +ve OPK Day 1 and not as much as that on +ve OPK Day 2. See it was tough for me to get it with cm and cp.