Maybe you'd care to elaborate? I don't see how your reply addresses the original comment that an additional app would be required.
No, my comment doesn't. But it's tiresome to constantly read these posts that clearly are based on that the person has no knowledge in the matter. It's not on me to educate these persons.
But ok, here we go:
Devices that supports HomeKit can be controlled from any HomeKit enabled app, at least the basic functions. For example, you can get temperature readings from your Elgato Eve Room from the "Home" app, you don't need to use the Eve app. But some functions may not be available, such as getting a reading off the Eve Room regarding air quality. For that, you will need the Eve app. That's not something that's limited by HomeKit though, it's a limit set byt Elgato.