I watched Greta and concluded that Jossie was stating that Joran's story in which Deepak went back to kill Natalee was incorrect because they were all seen together between 2:30 and 3 AM, looking quite suspicious. I think that his point was that the new witness brings the Kalpoe brothers back into question for their part of the crime. The witness' story is easily corroborated and thus, Jossie must be feeling pretty good about his story.
Well I personally think that they all took their turn with her on the beach. Now...did one of them kill her or did they wait there for someone to come and pick her up and take her off the island? No body, no crime. (they don't count rape as a crime apparently). That would explain why they were sitting in a car near the beach between 2:30 and 3:00 am.
But, Joran could still say that after they dropped him off, that one or both of them went back. There was/is still plenty of time for that.