Why “flong”?
A flong is the paper mould used in stereotype printing. You press a sheet of pulped paper and paste onto a forme of set type, and it takes an impression of the whole page. Once it dries you lift it off, and it becomes the mould you cast a metal printing plate from.
So the flong is the impression taken from the original type — portable, reproducible, and not the type itself.
That is exactly what this tool does. It takes an impression of source code that you can carry off and share, distinct from the code it came from. You cannot compile a flong. That is not a limitation being glossed over; it is the definition.
The word
It comes from the French flan, the pastry. Stereotypers coined it because the layered paper-and-paste mould looked like one.
The same process gave English another word it kept. A stereotype plate was cast from a flong so that identical pages could be printed again and again without resetting the type — which is how stereotype came to mean a fixed impression, repeated unchanged.