I think your writer's claim is a bit of a stretch. There's no risk of confusion and it's quite hard to see what other meanings could be reasonably construed.
The best I can think of is this:
Do not use the elevator [ever], because there might be a fire [that you don't know about].
Although the phrase "IN CASE OF FIRE" clearly means "if there is a fire", in general, we can say "[just] in case" to refer to an event that may already be the case, but where we don't know for sure.
For instance: someone in your household brings a guest to dinner and you don't know their dietary needs. You might say:
In case you're vegan/coeliac/observe kosher/halal, I prepared a plant-based gluten-free lentil dish.
In the same sense:
In case a fire might be happening that you don't know about, you shouldn't use the elevator.
or, for short,
IN CASE OF FIRE, DO NOT USE ELEVATOR.
It's a totally unreasonable interpretation for an Earthling to make, but it is at least logically and grammatically plausible.
Edit Owen's comment below explains it very clearly. Often, "in case of X" means "because X might happen", not "if X should happen".
"Don't eat honey outdoors on a hot day, in case of wasps!"
That doesn't mean:
"If there are wasps, you shouldn't eat honey outdoors on hot days."
"You should never eat honey outdoors on hot days, because wasps might attack."
Because we are familiar with the expression
"In case of fire, do not use elevator"
we know that it means
"Don't use the elevator if there is a fire"
but if we were seeing the phrase for the first time, we would probably take it to mean
"Do not ever use the elevator because there might be a fire".