I said I’d understand if you talked about the engine of The Witness. I think that is a fairly common colloquial expression. Engine is a fairly fuzzy concept at best. That you think there isn’t a different word like ‘tech’, ‘framework’, ‘libraries’ or the myriad of alternatives is by the by.

For example some of the framework for The Witness was ported to Jai for the sokoban game Thekla is working on.