I think that there's a common thread that underlies the development of intelligence in both humans and Portia spiders. We both evolved in an environment where we were at a sensory and physical disadvantage to our prey. We both relied more on understanding the behavior of our prey in order to hunt. In the case of early humans, we adopted a style of hunting known as persistence hunting.

Persistence hunting ( https://en.wikipedia.org/wiki/Persistence_hunting )is a general hunting strategy in which a hunter chases their prey over a very long distance(15-30 miles). Eventually the prey becomes weak and succumbs to exhaustion. On its own, that's not very special -- both dogs and hyenas adopt a very similar hunting strategy. The key difference is that humans lack the extremely sensitive sensory abilities that are used by other persistence hunters.

In the absence of such senses, early hominids had to predict where prey would go and accurately pursue them over very long distances based on very small amounts of visual evidence. In short, early hominids hunted by simulating the minds of their prey. If you look at https://www.youtube.com/watch?v=826HMLoiE_o (a documentary on persistence hunting), you can watch some Kudu tribesmen literally simulating where an antelope will go.

It certainly seems to me like lucking into an evolutionary niche where you get caloric benefits that are directly linked to how well you can simulate the fairly-complicated minds of your prey is pretty much a recipe for extreme selective pressure in favor of general intelligence. The story is a little different in the case of Portia, given that they are apparently at a visual advantage to their prey. However, I think the examples mentioned in the article make a strong case that Portia's comparative hunting advantage is in planning the best attack method based on the behavior of a given type of prey.