It's also mentioned in the Taint books within LS, where it's stated to be where the Taint originates from. There are a few old forum posts about it but the details are all really vague. In any case, I think there's good evidence that it's a real thing in-universe, but generally we don't know much of anything about it :3
Serenity's Taint is real, though I was referring to how Sycor and Cheriza believed they could absorb it from Serenity Horton. I'm not actually sure if it's possible to absorb the Horton Taint's power unless you're a part of that family.