What they achieved: automatically generated good code.
What they claim: automatically generated code that is revolutionary and an improvement on the state of the art.
And as another commenter noted, superoptimizers are also already a thing: https://en.wikipedia.org/wiki/Superoptimization
There's also automatic searching being done on faster sorting networks that actually recently produced better than state of the art sorting networks: https://github.com/bertdobbelaere/SorterHunter
Edit: compiling the hunter code Right Away and hopefully in some weeks I'll have better networks. Selection networks are even harder to find optimizers for, hopefully one can hack this new thing to get some.
But, at its core, this is really a RL paper. The objective is to see how far a generic approach can work while understanding as little as possible about the actual domain. After AlphaGo exceeded expectations, the question becomes: "What else can RL do, and can it do anything actually useful?", and this paper seems to suggest that it can optimize code pretty well! I'm really not sure they are self-aggrandizing in terms of impact. The impact of an approach like this could potentially be very large (although I'm not saying that it actually is, I don't know enough).
But ya, discovering a new compiler optimization automatically is kinda cool.
Is it really? I've heard of a few "better sorting algorithms" and it's never meant that in my experience.
There are already superoptimizers who use genetic algorithms to find the most optimal code sequence for small easily verifiable tasks. That is also a form of reinforcement learning in a way