I would assume the statement that more developers use Metal than Vulkan or DirectX is based on the large amount of cheap mobile games in the App Store. Well, not quite right in that case as most of these games were created with tools like Unity, which allows to create games without actually writing Metal (or Vulkan or DirectX or ...). These are very basic games, likely not optimized or implemented on a graphics API level. So while they're running on a device and even use Metal, that's all happening under the hood. It's a very different story for AAA games that are much more expensive to create.
In a similar way, apps can be created with Xamarin in C# and .NET and run on Android and iOS without the need to write Swift, Java, or any other code. Optimal? No, but works perfectly for basic apps.
EDIT: While I'm at it, GNM/GNMX is similar enough to DirectX, that anyone with DirectX experience will feel right at home. So while there are differences, Windows/Xbox/PS is somewhat of a large ecosystem that allows easy target switching.