This isn't actually accurate because, if you are a game engine, you can't just have a Vulkan-only implementation. There's plenty of old computers that don't support Vulkan. PlayStation doesn't support Vulkan - better use GNM, GNMX, and PSSL. Xbox only uses DirectX, which has both high and low-level variants, no Vulkan there. iOS is Metal only again. You could use Vulkan on the Nintendo Switch, but NVN is faster and the preferred method.

You might get away with just Vulkan if you were targeting only Windows, Linux, and Android on hardware that isn't more than roughly 5-6 years old. But Linux is a small percentage and they've got Proton, so as long as you don't want to port to Android and not iOS for some reason, why not use DirectX?

"lowest common-denominator API that everyone else supports" - Outside of the PC, Vulkan isn't really that broadly-supported. Yes, you've got Android and Nintendo Switch, but those are only individual items in their respective categories and don't make sense to support alone without the rest of their categories (smartphones and consoles).