Coreboot by itself is initialization firmware only. That means, it doesn't provide any callable interfaces to the operating system or its loader. So you cannot ask coreboot to load a block from disk. That's were BIOS, OpenFirmware and (U)EFI come into play to fill the gap. They don't define the firmware, but its interface.
I haven't read the article, but I'm quite sure that they're using SeaBIOS - running on top of coreboot - to boot Windows. In this setup, coreboot performs hardware initialization and SeaBIOS provides the required BIOS routines for Windows to boot.
So why not (U)EFI, you ask? Well, it just takes someone to place an EFI implementation on top of coreboot. I think GnuFI used to be able to run off coreboot, but I think the project is dead. TianoCore is probably a better option. Actually, I know that TianoCore is the better option and that it can be done, but certain legal obligations prevent me from porting TianoCore to coreboot at the moment.