The chapter doesn't at all feel random to me, the tension between Kabuto and Madara was gonna have to burst at some point and a new explanation of the technique needed to happen at some point. Like most all of us assumed, it was indeed the 4th that Oro tried to summon, we also know that you can't seal anyone who's soul is not reachable, which Oro didn't know, and you need a substantial amount of their body or blood to summon them.
Seems Jiraya was too far down in the ocean to reach and I guess since Kabuto was busy leading the leaf to the Madara's base that he didn't have time to go to the site of Kisame or Konan's death. I do however have one clarifying question, why did Kishi have the 3rd stop Oro's last summon if it wasn't gonna work anyway? I guess the 3rd thought he stopped it but didn't and it was meant to make us think that it wasn't just random that it failed.