First off, in which book did you read this?
Nonhuman intelligence is difficult to gauge. The study of animal behavior only paints a limited picture of what an animal is thinking or experiencing. Most of the time, you have to look at the development of the brain and ganglia to get an idea of what the brain may be capable.
No offense intended, but I'm not sure why you lumped monogamous mating in with intelligence. While is very uncommon in invertebrates, its not that common in vertebrates either. If you think about it, its not even a good idea from a reproductive point of view.
Second of all, I would have to disagree wholeheartedly. Don't get me wrong; stomatopods have a massive pedal ganglion, and the ganglial development shown in the eyes and antennae are pretty impressive (I've just spent the better part of the day looking at slides of porcelain crab and Pandalus cross sections), but the brain structure of octopuses shows a great deal more development.
For example, octos have larger cranial nerves, and bundles of ganglia which form a three lobed brain. I don't know how much MRI information has been done on octos, but, there is evidence of brain activity including problem solving, individual recognition, and massive amounts of sensory processing. Also, the sheer amount of neurons and fine motor control you see in the cephalopod requires a lot of brain power. Inteligence is generally the by-product of this type of cranial development.
So what book was this, anyway?
Sushi and Sake,