Ok well technically it is not really working, but the system does boot up all the way to the BIOS and memory checks, but it does not go any further. Famous overclocker Der8auer went through a blind process to discover that there is a particular ID pin on EPYC CPUs that when covered allows the CPU to be booted up by a X399 motherboard.
The motherboard in question was the ASUS X399 Zenith Extreme. Now ID pins are nothing new, they basically tell sockets whether or not they should be powering up a particular CPU.
So does this mean you can use an EPYC processor on an X399 motherboard? No, just that the sockets and pinouts are the same. It might take some more work to actually get the CPU working on an X399 motherboard. Check out the video below.