There are not many HomeKit air conditioners—are there any?—I don't know and I don't care. When looking for air conditioners, I am typically looking for specific physical sizes and cooling capabilities, that have very quiet operation and other specific features. I would never want to limit myself to the one model on the market that supports HomeKit. Instead, buy the air conditioner you want that meets your specific needs, and then control that air conditioner with a HomeKit compatible controller.
What I Use
I have two Sensibo Air air conditioner controllers, and they work really well. They are small (smaller than the picture at the left implies) at only 2.5" square and under 3/4 of an inch thick. They easily fit in the palm of your hand.
The Sensibo Air connects to your Wi-Fi and uses your air conditioner's remote control commands to control the air conditioner remotely. The Sensibo system also has a website (and app) that can enable "smart" controls that monitor and adjust your air conditioner based on occupancy*, schedules, and current temperatures like the Ecobee thermostat, or you can chose to control them through HomeKit.
In addition to controlling your air conditioner, the Sensibo will expose its temperature and humidity sensors to HomeKit, which you can then use to automate your air conditioner.
Note: The Sensibo does not have an occupancy sensor, it uses geofencing with a smart device for its "occupancy" features. If you are using HomeKit, you have other options available as well.