There's been lots of discussion about the fact the captain announced there was no danger and the smoke was external. Do we know what the conditions were like inside the passenger cabin? I believe some (lots?) of smoke had entered the cabin, but not the cockpit(?), via the engines and aircon.
I can easily believe that a passenger seeing a fair amount of smoke, possibly increasing, in the cabin might think the safest thing to do is to evacuate regardless of any announcements that they may or may not have heard. I'm sure that the selective audio and visual span that some pilots have suffered from in past incidents is not unique to pilots.