Ok, this is based on an article from Nature and I'm just a random dude from the Internet, so take this with a grain of salt:
What they claim is what I've learned in materials science class 20 years ago.
Metals fatigue under cyclic loading because of micro-cracks. This effect can be quantified by the Wöhler curve, which very simply put tells you how often you can bend a piece of metal until it breaks.
From what I remember, it is well known that this fatigue reverses by itself slowly and depending on the temperature and type of metal.
It is true that the effect is negligible for steel at room temperature, which admittedly is the most interesting case.
But for other metals this self healing effect can actually be exploited at room temperature. The example we learned in class was the leaden rulers everyone of us owned back then. These are rulers you can bend to draw nice curves in technical drawings. They are made out of pure lead. If you bend them very very often in one sitting they can eventually break. But if you stop before and you leave them alone over night they will be like new.
At least that is how I remember the story.