Does anyone know what the theory is about octupuses being solitary? It seems like a strange place to end up for highly intelligent creatures. (Although, now that I think about it being smart and specializing in/in charge of your own private hunting ground doesn't really sound like such a bad way to live.)

Sociality is not the only (and probably not an initial) driver of the evolution of intelligence in animals. Ecological and particularly foraging competency in less predictable environments is another and likely more primary stimulator for intelligence.

Coordinating complex movements is another one that appears to drive intelligence.

Vulnerability in a hostile environment. Together with foraging and coordinating complex movement plans, it is hypothesized that predation pressure when combined with the loss of their shell drove cephalopod intelligence. Trading defense for agility and int would also have provided a competitive advantage when competing for prey.

The book I reference above says that octopus intelligence is a bit of a paradox due to their short life span and lack of sociability. Pretty much all other intelligent animals are long lived and social.