#3 sounds extremely obsolete. I don't completely understand what they mean, but perhaps they are thinking pre-C89, when it wasn't as common to list the args in function declarations, so the compiler could not always correctly convert the type?

I remember in the 90s some libraries (like X11 headers) would put the types of function arguments inside an ifdef to be able to compile in some archaic place.

Edit: or maybe they are thinking of varargs. This could still be an issue with printf today.