Demiplanes existed within the deep ethereal like the bubbles in the froth atop an ocean of raw, untapped potential and possibility. That was the 2e conception of the topic, with a lot of discussion and detail in "A Guide to the Ethereal Plane". Wonderful book. Prior to that, not a lot of discussion on what demiplanes were, their origins, etc. in the 1e material.
3e sort of muddied the conceptual definition by suddenly dropping demiplanes in the Astral as well, and making the deep ethereal entirely optional. Easy enough to come up for conceptual rationale for the Astral demiplanes, but given the depth of the 2e material, and the generally much darker and creepier vibe that the trackless sea of the ethereal deep gave off, I kept with that location for the demiplane of imprisonment (which was where it was mentioned in 2e as well).
As for directly linking big T to that demiplane, there were some hints in prior lore, and I built off of them, making the link between the two overt, adding in some flavor inspired by the Black Cyst from RttToEE.