This guys seems to have his Phantom 3 Follow Me mode working?

Yes...many developers are working on Follow Me implementations.  Likely this poster is showing a  Beta version of someone's application currently under development.  Shouldn't be too long before someone releases this sort of implementation, currently unavailable to the public.  This poster doesn't identify the application under development they are testing in their YouTube comments:  


"We are testing the flight mode Follow me on quadrocopters Phantom 3.  This video demonstrates the mode when quadrocopter sleduemy ahead of the object and follows its trajectory. There is also a mode of following behind, left, right and circled the object around.  Currently carried Closed software testing. Follow the news."

I'm sure a lot of people will be glad to have it become officially available from DJI - sooner I hope.