Almost positive that terraforming is tied to Gameplay Affinity and not Visual Affinity. So if youre playing as Riftborn visual, you terraform like a normal faction would: Sterile -> Fertile. Its the other way around on Continuum Sculptors because your colonization options are flipped along with extra buffs to population caps on Sterile planets(Burrows). I'll check next time im on to confirm this since i have a Riftborn visual faction thats far enough along to Terraform. If i happen to be wrong though, chalk that up as another "bug" to fix.

Oh, I am absolutely sure that terraforming is tied to Continuum sculptors. 

But the quest seems to assume otherwise. Or the quest doesn't actually check at all which planet it demands terraformed. At the time I got it this was the only planet in the system that was fertile.

The main reason I didn't outright put this into bugs is that I have no idea yet how to reproduce it - I suppose I should make another faction with riftborn/not-continuum-sculptors, or actually the other way around, and find sowers and check. And of course that I'm not sure if there's something else I'm overlooking. 
I suppose I could always blow up the planet?