What else was the Capt to say. It is either evacuate so that the cabin crew know exactly what to do or come back and have a group discussion over a cup of coffee in the galley.
SOP are there for a reason. Cabin crew yell so as to reinforce what is required.
Could also help in turbulence
In my company at least, there is provisions for precautionary evacuations. But these are planned evacuations with appropriate PA’s etc. beforehand and done in an orderly fashion. There’s no way this would have been a precautionary evacuation otherwise it would have been known in advance.