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A good question
First, it's the Ki Rebel Establishment, not the Ki Empire (yes, in the Siglerverse, alien cultures can also fragment, just like Humans, and are not always a unified race from a one-ecosystem planet ...). The KRE does not have the wealth of the Ki Empire. Even for cultures that have achieved faster-than-light capability, there are still going to be significant differences in tech, particularly at the military tactical level (In the first Gulf War, for example, the Iraqis had "jets" just like the USA did, and that wasn't much of a contest).
The KRE has just three planets, with a combined population of 8.4 billion. That puts it around sixth or seventh in total population among the fourteen major governments (the Sklorno Dynasty is number one, with 263 billion).
So that gives some information on current tech discrepancy. As for your question as to what happened to their tech if they could reach Earth 700 years ago? The clues are in the timeline in the back of THE ROOKIE hardcover, my friend. There is an answer.