Looks like it makes heavy use of the original binary.

https://github.com/OpenLoco/OpenLoco/blob/master/src/Interop...

I can answer this - The reason is pretty simple actually, and the other reply is correct: Right now, the way it works is that the original game executable is modified to run the code from OpenLoco - Replacing Main(). OpenLoco runs anything that's been reimplemented, but anything that isn't calls back to the original binary, which means to avoid unneccessary complexity, it should all be the same - 32 bits.

OpenRCT2, from mostly the same developers, (Disclosure, I am associated with them), did the same thing until the game was fully reimplemented, and then it very quickly gained 64 bit support.