Besides the fact the MSVC's target platform isn't really related to the architecture for dependencies being installed, I'm not sure VSCMD_ARG_TGT_ARCH is an appropriate variable to look at in the first place. It doesn't seem to be at all documented, and (from looking at the implemenation) it seems to be meant as how parse_cmd.bat communicates with vcvars.bat, dotnet.bat, winsdk.bat, etc (these are all sub-programms that make up vcvarsall.bat)

I think %PLATFORM% (which generally ends up set equal to %VSCMD_ARG_TGT_ARCH% once vsdevcmd\ext\vcvars.bat has done its thing) is might be a better variable that cross-compiling should actually look at when it wants to know what architecture MSVC will compile to.