I suspect that comment predates the existence of capi and is trying to avoid having some other actual calling convention like stdcall because the function pointer is actually invoked from some C code in the RTS. capi is a sort of pseudo-calling-convention in that it generates some C code and then calls that stub.
Have you tried using a capi defined funptr there to see what happens?
Monthly Ask Anything (July 2023)
Use this thread to ask any Haskell related questions which you think doesn't deserve a thread of their own. Ask away!