I don’t know how they are sure it came from a proto-planet beyond Jupiter. What happened to that body—did it become one of the moons of one of the outer planets?
It got turned into space confetti by impact or some other event we probably can’t know — *orrrrr* this particular bit (and probably many others) got blown away into new orbits by a large impact, and the rest of the planet is still out there, undiscovered. I’d guess the second one.
It could have come from another solar system, like ATLAS.