Early access videos can only be viewed via an embedded player in the Patreon post, and only after entering your Patreon associated email into the player to authenticate. When you hit play the embedded player tech does behind the scenes magic using the source video plus your personal info to create a new video to send in the response which has your watermark on it.
I mean, it's a pretty clever way to prevent leaks until early access is over. The fact that he eventually releases the original video after early access is smart too. If he never did that then there would be some super sweaty try-hard people trying to crack that video tech or painstakingly editing out the watermark. But now it's like "meh, EA will end eventually and it'll get leaked - not gonna bother".