Do you believe this is impossible? It's really not, because some people do it. See yourself:
Writing source code directly in assembly language may make your code faster, as you can write the CPU instructions directly, making some critical optimization faster than the compiler-generated code.
Of course, the statement above about speed is false, and sometimes it's hard to write a faster code than the compiler generates. And, as you can see, the submissions just contain the code generated by the compiler instead of being written manually.