It is? My memory of the Atom's origin is he finds a metor and make a lens from it that enables him to shrink things.
My point is, the only way the meteorite wouldn't sink to the center of the Earth like a cannonball through quicksand was if it was already an alloy/admixture with only a small percentage of degenerate matter in its makeup. And even that's highly unlikely. Basically, as with most things in DC or Marvel, this is a fantasy treatment of a scientific concept.
Although I remember now that they didn't call it "dwarf star alloy" in the comics -- I got that from Doctor Who. I always get that mixed up with the Atom's dwarf star matter or whatever they called it.