To get a C-program to run, even an empty "int main(){ return 0;}" needs some supplemental code to set things up the way your main() function expects.

This supplemental code is what the github repository provides. You can then write very small, statically linked, programs without pulling in most of the C-library and other conveniences, and you will only be able to use the "raw" interfaces your kernel provides.

E.g. you'll have first to ponder what the "write()" system-call actually is, then use write(1,"Hello\n",6); instead of the wrappers and convenience of the C-library such as printf() which of course additionally gives you formatting, buffered I/O, ...).