This is impressive, and concerning. This is only going to get more convincing.

A method like watermarking doesn't make sense---someone will just develop a system for their own use that _doesn't_ use watermarking. There's no choice but to develop better forensic tools.

Inevitably, one day, those too will fail. If you want to know that something legitimately originated from someone, we need to do so cryptographically. If you want to know that something was legitimately recorded and unaltered by the legitimate source of the video, well, there'll always be a way around that.