For ESP-IDF and other CMake-based projects, the platforms/configurations are just a way to quickly pass different parameters to CMake. Normally, the configuration translates to the CMAKE_BUILD_TYPE value, and the platforms can be defined arbitrarily, with each platform having its own CMake flags.
This mechanism is completely separate from the regular MSBuild-based platforms and configurations. It is not designed for building ESP-IDF code for x86 and will not work even if you manage to somehow hardcode the x86 platform.