[anderiv]

Your intuition is correct. The story as told by the parent is likely just due to a misunderstanding (or just lack of knowledge) of 1) the pilot's words over the PA and 2) standard procedures used in commercial air travel.

[cguess]

Especially in emergencies. Lights are turned off so passenger eyes can get used to the dim light and see the exit lights (which are not bright) better.

[bonestamp2]

... and if it's dark outside, their eyes are adjusted to see in the darkness once they've evacuated outside the plane too.