Cuz Turtledove
They'd have been absolutely screwed if they couldn't capture industrialized military areas, that's a wonderful point they should have sent the colonization fleet in tow with it, armed their ships they had their planet armed just in case, they know it's a possibility and keep it stocked with orbital assets.
Well the point is the Race never imagined the Conquest Fleet would ever need to capture industrialized areas in the first place due to how primitive Earth was when they scouted it via probe before the fleets were launched. Further they couldn't capture the "industrialized military areas" in the OTL, in fact it's a plot point that much of the territory they captured was the less developed portions which hindered the Race's attempts to use native industry to supplant their finite supply of munitions compared to the industry of the Big Five who were successfully resisting them.
Tacking the Colonization fleet with the Conquest fleet not only would risk endangering the completely civilian population needlessly but would have added 80-100 million mouths to feed with a preindustrial society. So the Race methods was likely the best option.
Similarly there was no point to equip their ships with weapons since by that point of the story they had never encountered a peer opponent. Their most advanced conquest, IIRC, had only just developed radio when the Race annexed them so it just never would have occurred within the framework of the story.
Now part of the fun of these kind of Crossover is taking the Race out of their comfort zone and forcing them to, like a Big Ugly, make snap decisions.
Like imagine a crossover where the planned Conquest of Earth is postponed because the Race discovered a millennia old Ork Kill Kroozer in the crater of an airless moon with the dead, mummified remains of the Mob boyz onboard sparking fear in the liver of the Race that technological advanced races exist elsewhere in the Galaxy and, quite clearly, they are dedicated to warfare bristling with weapons both savage and incomprehensibly advanced. In response the Race cut the ship up and ship it back Home to be poured over and studied in a frenzied Manhattan-scale project that in only a few short centuries will manage the herculean task of starting to produce a modular, first-generation Warpship for colonization and military purpose opening the 2nd millennium galaxy up for them like never before.
Initial attempts go poorly as they find out outside their local star cluster is teeming with alien empires and horrors through not absolutely lopsided. What the Race lacks in raw power or experience they make up with having robust, well-refined technology they understand. Their ships can't stand toe to toe with an Ork Kroozer but they can engage at light-seconds outbound with guided missiles in giant fusillades until they run dry of ammunition or the Orks cease being a threat. Ork hordes will always overwhelm them on the ground closing the distance until they pin the Race's army in place and demolish it their use of precision guided munitions, mobility and well stoked nuclear arsenal at least ensure the Orks are bloodied.
With all of this having knockdown effects on the Race from them developing a permanent military caste to them, once they rediscover Earth, using Ginger in conjunction with a breeding program to explosively increase their manpower base. Given enough time they might even go the eugenics route trying to breed a larger, stronger faster soldier strain by selecting for those traits
