My basic understanding of orbital mechanics tells me that if parts of 04a got out of Threshold's gravity well, other parts had to fall deeper. If Halo was under tension, the fragment in Nightfall could have been thrown free when the ring snapped, and the recoil would have punted the rest into Threshold.

Somebody should set it up in Kerbal and get back to us.

Also, the news that the Halo fragment is orbiting a sun tells me that the Soell system just got upgraded into a binary star system. Because there's no way a Halo fragment traveled several light years in six short years.

Alternatively, this is the fragment that the Vessel crashed on. The Pillar of Autumn's detonation woke it up, and it jumped into slipspace, taking a Halo fragment with it.

Oh, wait, the full Nightfall panel indicates that a part of Halo initiated an emergency Slipspace teleport. A'ight.