Rather they [GDW] were developing the whole [of the OTU], with the publications of the rules, boardgames, and JTAS articles being essentially snapshots of their train of thought at the time of publishing.
They may not have had envisioned such a whole at the very start, but that shifted, and reasonably quickly, for both commercial and, I assume, personal interest of the developers. That is, an 11,000 system spanning Imperium didn't just happen, They(tm) wanted it that way. When that concept became a gleam in their eye, anyone can guess.