That's a nice list, quite complete. Many console exclusives also use custom engines by the way, e.g. Decima for Horizon:ZD, KillZone and Death Stranding, Naughty Dog has their own engine (don't know the name), etc.
The OP's point was that the companies that make these engines can afford to invest in supporting an additional back-end API though. I think it's hard to argue that any of the companies that develop these engines would not be able to also add a Metal back-end. Many of them already work across a pretty wide range of back-ends anyway. Xbox One, PS4 and Switch all use completely different API's, for example. I think most of the work is not in adding an additional backend like Metal, but in tuning the back-end for some specific piece of hardware (NVidia vs. AMD vs. mobile GPU, etc).
Whether companies are actually willing to invest in a Metal back-end remains to be seen, but considering many of them license their engine for commerical use, I would be surprised if the major players will simply ignore Metal.
I tend to agree with Jonathan Blow's comments on Twitter, that the low-level graphics API should be just that: as low level as possible, small, focussed, and not actually intended (but still allowing!) to be used directly. Engines or higher-level API's can be built on top of that, with the option to dive down to the lowest level when needed (which will probably be a rare occasion).
DirectX will definitely not be this API because it is Windows specific. Likewise for Metal because it is Apple-specific. Blow appears to be of the opinion that Vulkan is also not moving in the right direction, because it is becoming too complex, and trying to be too many things for too many applications at the same time.
If true, in a sense, it's not that surprising Apple is doubling down on their own API. I think they should consider making the API open-source though, and develop something like MoltenVK (but the other way around) for Windows/Linux.