Platforms means nothing for assembly. It is used and generated by Visual Studio Configuration Manager for seperating different sets of build settings (configuration-platform pair).
C++/CLR projects have default configurations that use "x64" as its platform name, so a platform called "x64" other than regular "AnyCPU" will present in Configuration Manager.
In OpenSense, Platforms attribute is also set for C# projects to uniform the representations of C# and C++/CLR x64 projects in Configuration Manger. Removing this attribute will not change any value in output DLL files.
PlatformTarget makes the resulting assembly targeting to AMD64 instead of MSIL, which is written to project output DLL files.