Isn't this a bit like cold welding? [0] Unless something is in-between them or some force is pulling them apart, there's no big reason for two exactly-fitted pieces of the same crystalline metal to _not_ join together. It's not like the atoms "know" which side they belong to.
Actually... [tap tap tap] It looks like that's explicitly in the original Nature title: "Autonomous healing of fatigue cracks via cold welding." [1]
[0] https://en.wikipedia.org/wiki/Cold_welding
[1] https://www.nature.com/articles/s41586-023-06223-0