Game engines need to support multiple APIs anyway. DirectX for PC/Xbox, a different API for Playstation, and yet another API for Nintendo Switch. From what I've heard from people in the industry is that you really want to use the platform specific APIs because they're developed by the same company who built the hardware, and as such there are a lot of exposed hardware-specific APIs and tricks that can increase performance by a significant amount. And those features are guaranteed to be there for every system that it will be played on. PC, and by extension Vulkan, doesn't get that kind of hardware access since it's made to be generic and usable across a large range of generic hardware that may or may not have those same features. So while you may be able to target Vulkan on Playstation and Nintendo...performance is likely to be terrible compared to using the native APIs.

Adding a Metal renderer isn't the big deal. As others have said, the deal is that they won't get enough revenue from Apple sales to justify the development and support of Apple hardware.