The testing was, for obvious reasons, done in a simulator. But they don't say how they'd spoof the transmissions in reality, unless I missed something. With TCAS you're going to have to simulate a transmission coming from a specific location, it all works off the change in time of successive received signals. I'd have thought that's the trickier bit.
ADS-B woukd be a bit easier to spoof as it sends a position rather than just worrying about how quickly things are approaching.