Earlier quoted context omitted.
The Vulkan/D3D12/Metal port is done on the side, and we don't want to disrupt the rollout of the WR-powered Firefox onto wider userbase at this time.
So it's planned to be focused on once WebRender is out and enabled everywhere? I see some work here: https://github.com/szeged/webrender/issues/198
Yes, that's the idea, although it's not officially approved by the product management yet. And that link is correct.