I think what
Paitryn was trying to say is, it is indeed an old
game engine called TK17. A standalone expansion of 3D SexVilla if memory serves right. It is a game engine hybrid with minimal model & animation tools included. Was one of the first of its kind. Think Honey Select grandpapa; a game engine of the early 2000's which is now modified with a graphic hook (hook 5) to kinda look updated. The current version is a highly modified version by modders that the dev uses. Could even say it is another expansion of the expansion by modders, along with content creators assets downloaded from that source.
Treasure of Nadia uses the same game engine as this game but the model/animation is not TK17 but Daz3D. Hence the graphics are modern and updated.