While it’s also useful for SpecialK compatibility I’m a bit curious as to the double API nature of the engine too, Vulkan should be covering Windows 7 - 10 plus cross-OS support like Linux if the developers want to attempt something of that scope whereas D3D11 is limited to 11.1 sort-of on Windows 7 via the platform update and then well Windows 10 and various builds more than Windows 8 or 8.1 which is fading a bit.
D3D11 - D3D12 support would be more expected and using D3D11onD3D12 or similar to start into D3D12 over a pure / native implementation (And restricting to Windows 10’s newer builds only.)
It’s useful though but a bit surprising as it’s not very common.
Croteam and Serious Engine as one other engine doing this, Unreal I suppose but rarely in shipped configurations (Especially with Vulkan.) just that the core engine supports these API’s because it’s well developed by a dedicated team for it.
(Croteams prior senior engine technician seem to have left the studio last year too but the engine is still going so they seem to have a replacement at least with similar skill.)
EDIT: Different practices and what’s best plus the more modern VLK (And D3D12) API’s and what these allow for but if they can maintain and keep both of them supported and utilized to best effect it’s not like it’s a issue just a bit surprising really for the extra resources and effort needed.