You honestly don't think they are rehearsed? Even if the flow of attacks isn't completely rehearsed, the way uke's are directed where to attack and the measured way in which they do so is practically the same thing. Though I'd still be they're hardly spontaneous.
I don't see anybody being directed where to attack once they are going at it. Each uke looks like he is sticking to one attack, I think. No sure what you mean by measured...they are committing themselves, but you can see Doshu start to move before their attack really gets power behind it, and at that point they move into getting ready to take a fall. It's an embu after all, not a test or a fight.
And this is hardly the same thing as a rehearsed sequence of moves. IF it was rehearsed they could pull off some really dramatic looking things, or brutal looking things such as in a koryu embu.
So no, I do not think this is rehearsed. I will grant that uke may be being queued as to what attack to offer. And I will grant that the ukes and Doshu probably have experience training together and the ukes have been generally whipped into shape over time. But I do not believe Doshu is putting a significant amount of contrivance into what he does with the attack. He just lets it flow. And he doesn't put any fancy flourishes or anything in, it is just basic, pure Aiki.
Does anybody who has taken ukemi at one of these things have anything they add here? Does your teacher spend a couple of nights or weeks before the embu working out what techniques they are going to do and in what order?