I don't know what the documentation looked like at the time you encountered this but based on a dummy export apparently working around the problem and the current documentation, you probably missed some requirements stated in Detouring 32-bit and 64-bit Processes. Unfortunately it says to do things without explaining how the pieces fit together.
Detours expects your helper DLL to export DetourFinishHelperProcess with ordinal 1. This isn't a function you define yourself. It will get compiled into your DLL as a result of calling DetourIsHelperProcess in your DllMain function as shown in the Using Detours sample.