Finally got this scene.. 

You have to have Astarion as your only companion while playing as Dark Urge. Dismissing everyone else from your party won’t work, he has to be your only companion, period. I’ve heard it’ll work if everyone else is dead though, but I’m not sure for myself.

Not important I think. But it's fun to watch his true reaction without any other companions.

It’s also extremely unlikely for someone to get this without doing it intentionally since he has to be your *only* companion. Everyone else has to either not be recruited yet, or dead.