Not true. Crossfire and SLI, are both methods for running multiple graphics cards on DX11 and earlier, require explicit display driver support to function. As far as any DX11 (and earlier) application in concerned, there is only one display device present regardless of how many GPUs are installed. It is up to the display driver then to manage the workflow. As Black Ops 4 is a DX11 title, crossfire support would have to be implemented driver side by AMD.
MultiGPU (For DX12 and Vulkan), exposes all display devices to the application. Allowing the developer to explicitly program how rendering is handled across multiple GPUs. This support then, must be implemented on the engine side, and thus fully the responsibility of the game developer.
