Kubishush does have a population of 9 billion, so 10% is still a large population, almost a High Population world in its own right.
Given the Gl'lu obsession with redundancy, and constant need to rebuild their settlements, I imagine they provide a lot of very enduring equipment, very robust and safe vehicles, and simple industrious effort (they're used to constant maintenance/replacement). That many people involved in continuous large-scale reconstruction, I imagine they would have to count by default as a major industrial centre.
Plus, the average Imperial citizen is probably biased toward the 10% of Gl'lu that trade across the sector and handle the more advanced technology, so having the sector overview in 2nd edition focus on the "Gl'lu as Imperial citizens and active traders" makes sense to me.