The devs (at TFP) are not writing the engine, they are using Unity (did you mean interface code to Unity instead ?)
But even updating to newer versions of Unity (which they do regularily) does only get them bugfixes, not any advanced features because they use a legacy engine inside Unity from which there is no upgrade path to the modern engine.
Official word from Faatal a few months ago was that they would have to rewrite their interface code so much that it would delay release too much. So they have no intention to do that for this game.