Maybe the driver is injecting the FF emulation shaders that it puts in for any D3D9 call, that would bind a VS at least that could just be a pass through shader seeing no MVP is set either. Which then would mean that as long as you keep the vertex coordinates in screen view you will see the triangle on screen. This would off course only work with a driver that still has support for D3D9.
This is all speculation of course.
Have you tried doing a VS graphics/Renderdoc/Intel Graphics analyser/NSight capture to see what the GPU is actually doing?