“I think the theory is that with such technology they would have destroyed themselves if they were not peaceful by philosophy.”
History is replete with examples of warrior societies enslaving the toolmakers to provide the technologies the warriors are incapable of producing themselves for the conquest of even more cultures. The Klingon Empire is a fictional example of this principle based upon a wide variety of actual historical cultures such as the Arabs, Mongols, and others.
True... at the same time, they didn’t extend themselves so far beyond their home bases that they couldn’t get supplies. Doing something at interstellar removes is a whole different story, especially when warp travel isn’t actually possible.