Fridge Brilliance: The Zhirrzh's weapon lasers are said to be so much more advanced than human or other known alien efforts that Pheylan describes them as 'impossible' in the initial battle. Unsurprising, considering they will have used lasers much more extensively for communications when Elders weren't available, as they can't use radio, so will have had more incentive to research and improve them in general.
Fridge Logic: The Zhirrzh rule over several races that previously 'attacked' them and keep them at a low technological level, with scientists occasionally studying their planets' ecosystems and having contact with them. So in all that time none of those races ever mentioned to them that radio isn't meant to be a weapon?
Maybe they ended up slaughtering every member of each species who knew what radios were. We see that at least one of those species is completely subjugated and living barely-civilized lives, not allowed to gather...
Everyone who knew? I think it's safe to say that just about everyone on the planet in our society knows what a radio is for, even if they can't build or operate one. And in five conquests there was no formal surrender during which a conquered race said "wait a minute, what is this 'attacked us with radio waves' business in the surrender agreement about? Radio waves aren't weapons."?