The demo of the game is an executable that self-extracts itself into a temporary directory and then relaunches from there, and therefore ReShade won't be loaded (since it's not installed to the executable in that temporary directory, but the original one). You can just extract the game manually though (open the Monolith.exe with e.g. 7-Zip and extract it) and then install ReShade to the new Monolith.exe you get from doing so. With that I get ReShade to load as expected (as d3d9.dll).
I'm assuming the Steam version behaves similar.