This works if you are using Windows or another operating system that ignores case on its file system. It doesn't work on Linux and other Unix flavours, because they do care about case (by default).
When you add an include statement like
#include <any_file.h>, then the C-compiler will ask the operating system to open file
If your operating system doesn't care about case, then it will open any file that matches the pattern. If it, for instance, finds a file named
aNy_fILe.H, it will open the file and present it to the C compiler.
The C-compiler will interpret the content of the file - this is the case-sensitive bit of the C language.