I do not buy this argument.
Its called a documented function.
The developers know the function's inputs and outputs and what it does.
That's all they should need to know.
Its no different to functions in the libraries of whatever programming language you are using.
Devs just do their coding based off the function signature and docs. They know what goes in, what comes out and what the function does.
How many developers do you know who've gone back and read the source code of the function ? Assuming its open-source anyway and not a OS API.