I get that they're saying the diamonds could only have been formed within something the size of planet, which subsequently broke apart in the "early days" of the Solar System.
I get that Saturn and Jupiter are so massive relative to other Solar System bodies they've marshalled fragments into the Asteroid belt, Kuiper belt, outer disk. And that they're still disturbing those regions enough to occasionally pull things into a cometary orbit/become meteors.
This putative planet must have scattered everywhere when it first broke apart/lead to comets and meteors all over the place. But why/how did this fragment wait until 2008 to come to Earth? Where's it been for billions of years? Why didn't it either crash into something a long time ago; or form itself into a stable orbit?
Possibly related, as in: why haven't meteorites originating from early-formed planets all long gone extinct?