Chipzilla is apparently doing all this using emulation running on an open-source conversion layer known as "D3D9On12" from Microsoft.
The conversion sends 3D DirectX 9 graphics commands to the D3D9On12 layer instead of the D3D9 graphics driver directly. Once the D3D9On12 layer receives commands from the D3D9 API, it will convert all commands into D3D12 API calls. So basically, D3D9On12 will act as a GPU driver on its own instead of the Intel GPU driver.
Microsoft says this emulation process has become a relatively performant implementation of DirectX 9. As a result, performance should be nearly as good, if not just as good, as native DirectX 9 hardware support.