It is a principle and a very good one, if a difficult one to follow.

The point is not that it's difficult to follow, but that in some circumstances it would be wrong to follow it, as the "Fire" case exemplifies - circumstances where an utterance that might just be a word in some situations takes on the qualities of an action, and an action that is potentially very dangerous.

Perhaps it is best seen as a guiding principle rather than a ruling principle.

Incidentally it pretty definitely wasn't Voltaire who said it, though that doesn't make any differrnce. Quotes like this atr always getting fathered on people who are famnous enough to help them stand up.