Technically speaking as XMM is a .net Application, there is nothing like x86, x64 native code.
.Net Programs use a Just-In-Time-Compiler to create Native Code after you start the program on a PC. So .Net Programs can be universal x86/x64, but only if all Components are universal and external native dll's, ... are available in both x86 and x64. Since many dll's,... aren't available as x64 a .net Program can be forced to be Compiled in just one way. In the case of XMM it is forced to compile as x86. I think the main reason is because the Database Engine Jet (Access) from Microsoft is only available as x86 and will never be available as x64. The Jet Engine isn't developed anymore by Microsoft.