Why would the human empires even bother?
A pre-modern empire doesn't go and conquer territory simply because it can. Administering a conquered land is expensive. You need to clear the forest, build roads and aqueducts, defend the frontier, administer justice, ensure public peace... So that the empire does not seek to expand into a new territory if it cannot extract sufficient tax revenue to bother with the hassle.
In short, there must be something there which the empire wants. The Roman empire stopped expanding in the 1st century CE, because there was nothing around it worth conquering, except the Persian empire in the east, and conquering Persia was beyond their power. But the vast northern forests, the great eastern plains and the endless southern desert were not worth the effort, and they made no attempt to conquer them. Similar for the Sasanian empire; to the west was the Roman empire, and that was attractive, but they were not strong enough to grab it: but to the north and the east, they stopped expanding. And between the two empires was Arabia, and yet neither of the two bothered to conquer it.
The point is, a pre-modern empire is severely constrained in what it can really grab and hold.
Distance is also great demotivator: for example, the Romans knew that there was habitable land somewhere south of the great desert which marked their southern boundary, but it was simply too far away. A pre-modern empire cannot really project force that far away.
Another factor to consider is manpower. We are accustomed to think in terms of the modern world, where we have an abundance of humans. But this is a very very recent situation. Throughout the vast bulk of history, population densities were very much lower than today. A pre-modern empire faces hard constraints with respect to how many people it can engage in conquering faraway lands; generally speaking, pre-modern empire vastly preferred conquering lands which already had lots of people, because having lots of people meant a vibrant economy, possibility to extract taxes, the ability to organize those people to defend the new boundary, and a reservoir of workforce for the roads and aqueducts...
For an appropriate example, consider the boundary between Roman Egypt and the Aksumite Empire. Aksum was theoretically much weaker from a purely military point of view; but the boundary (at the Elephantine Island, just north of the first cataract of the Nile) was stable for many centuries -- simply because there was no way at the technological level of the Roman empire to mount a serious offensive so far away, and because any reasonable emperor must have realized that even if Rome prevailed, the semi-autonomous rulers of a remote province would always be tempted to rebel and proclaim independence.
Or consider the Turkish empire(s). The Seljuk and then the Ottoman Turks could have annexed the great steppe north of the Black Sea at any time between the 12th and the 16th century. But they didn't, and they never even gave it a second thought. Why bother with it? It is sparsely populated, the inhabitants are poor and belligerent, the land is not suitable for the Mediterranean style of agriculture, there is a scarcity of water, and winters are terrible. So what they did is they maintained an arms-length sort-of-friendly relationship with the Tartars of the Golden Horde and otherwise left them alone.
For a setting much closer to modern times, consider the Great Spanish Mexico, officially known as New Spain. On the map, the Spanish empire included most of what it today the United States of America. But in reality, of course, they never even tried to administer the bulk of that territory; in principle, the Spanish empire was much stronger than the Indian tribes who lived there, but in practice Spain never had any motivation (and definitely not the man power) to go and explain to the Indians that they were subjects of His or Her Most Catholic Majesty and they should pay taxes. When the United States of America started to expand westward, they never met with any Spanish opposition...
So it is enough if the Aelurodryads live in a great forest or on an endless savannah somewhere far from the center of power. There is nothing there for the human empire to lust after: the catty elves have no gold to speak of, they are dirt poor by the standards of the empire, there is no hope of taxing them, and overall making them into a province of the empire would be a financial disaster.
