Asked Guy Haley on Twitter:
Tweet can be found here
The connection between Armageddon and Ullanor wasn't there when the planet was invented. Like a lot of WH40K lore, it just developed. Most of this stuff isn't planned out in such detail years in advance (10+ in this case), but we try to make it look that way!
This is his tweet
My previous answer from before Guy Haleys tweet:
Armageddon is part of WH40k since at least 1992 with the board game Battle for Armageddon:
Das Spiel Battle for Armageddon ist ein Brettspiel im Warhammer 40.000-Universum.
From the German Lexicanum on Battle for Armageddon
The game Battle for Armageddon is a board game in the Warhammer 40.000-universe.
First mention of Ullanor I could find was in Index Astartes IV from 2004. I don't own the book so I use the quote from the Lexicanum Article on Horus Lupercal:
As the Emperor and the Great Crusade marched on, Horus proved himself to be a tactical genius. He knew precisely which force to send and where to send it, showing no mercy to those that opposed the Emperor but sparing the innocent from unnecessary bloodshed. As the Emperor departed to find and meet with rediscovered Primarchs, Horus was left in temporary command of the Legiones Astartes and this helped prepare him for the role of Warmaster.
They cite Index Astartes IV
The transportation of Ullanor into Segmentum Solar was described in The Beast Arises: The Beheading, published in November 2016. I don't think they've planned this 12 or more years ahead.