Because all three are APIs which try to provide a lot more control over the GPU to game developers. And I wouldn't know of any other similarities that DX12 and Metal share...
But the point he was making was that there is no need for DX12 because Vulkan works everywhere else.
Right now Vulkan is only supported on custom Android 6 forks for Samsung S7 and NVidia Shield, Android 7, GNU/Linux and Windows on a restricted set of graphic cards.
Hardly a market to target for game developers that want to reach as much eyes as possible.
Just think how much more popular Windows 7 is compared to DX12. Xbox One is nothing in comparison.
Windows 10 users are already 45%, with around 41% having DX 12 GPUs + plus around 20 million Xbox One units.
As for market size of Vulkan, some S7 and NVidia shield as part of 10% Android 6, 0% on Android 7 until it reaches stable and of course those other 55% users that might have a Vulkan compatible card.
