The colonization cutscene shows an abundance of swamp trees and monsters

I assume it is sterile in a sense that it is extremely hostile territory and the population - and presumably most of food production - is confined inside the artificial habitats, making the colonization effort and habitation similar in nature to that on the other hostile planets.

Looking at it this way, it is fertile only for its own life, but sterile for the outside one, if that makes sense.

I think of them like Death Worlds from the WH40K universe. Basically, teeming with life, but anything non-native is going to have a real bad time trying to exist without dying horribly in any number of ways. Hence the low yields despite seeming abundance, and especially the ridiculously high approval penalty.

And large amounts of Yuusho recruits running around, slashing every venomous, poisonous and toxic thing they see ;)

Probably sterile by human standard, not local life.

IIRC it received that trait for balance between planets reason.

Swampy planets are full of nasty flesh rotting fungus and bacteria. 

Also Sterile doesn't mean lifeless in the ES2 game....it is more like, low biodiversity. You have species like the Pulsos, Amblyr, Kalmat, etc. who thrive on these inhospitable worlds.

There are also species like the Galvran who live on an Arctic planet with incredibly harsh  and long winters...