I can explain how this idea comes in my mind, actually I have managed to use CodeBlocks with UE4 and found 70% to 80% performance increased and the CPU lows down from 52c to 30c, but the only problem with this compiler you have to load all the files manually and compile to sea the result in the editor, it will not restart the editor on every compilation, you have to do it manually.
CodeBlocks IDE uses a powerful C++ compiler GCC and absolutely open source.
I think this will not be a problem if EPIC will take this IDE as a reference or template and build this further and optimize for UE4/5 …