Why do function names that are not part of the implementation still use the double underscore naming convention?

Aren't double underscores reserved for the implementation? I'm referring to something like this. People seem to ignore this convention all the time.

Here's the code:

signed int __cdecl upload_exploit() {
int device_type;
signed int payload_address;
int free_address;
int deviceerror;
char *chunk_headers_ptr;
...

Here is where the convention is defined:

In addition to the names documented in this manual, reserved names include all external identifiers (global functions and variables) that begin with an underscore (‘_’) and all identifiers regardless of use that begin with either two underscores or an underscore followed by a capital letter are reserved names. This is so that the library and header files can define functions, variables, and macros for internal purposes without risk of conflict with names in user programs.