It is a very interesting implementation. It works based on the homekit-python library
this library can create it's own HomeKit Controller instance, think of it as another iPad or Apple TV not linked to your iCloud account.
You can then discover any IP based device on your network that has HomeKit support and link it to that controller created by that library. You don't control it through your phone directly. You actually have to link each device twice: To your phone and to your controller and you have to do that for every device you wish to control.
Let's say you change the temperature on your Ecobee, here is how the flow is:
On your iPhone change temperature in the Home App ----> Home app sends message to Ecobee to change temperature ---> Ecobee sends message about temperature change to this Controller ---> The controller informs HASS
Technically, it would be possible to get something like this done for Hubitat, but it is quiet some effort. We would need to program a new "man-in-the-middle" application that implements this controller. This application also needs a User Interface (website, console, etc...) as you need to interact with the devices during the pairing.
a) you need to select the device you want to pair
b) you need to enter the pairing code that is given by the device
Then you would need to develop an interface to Hubitat, a Hubitat app that interfaces with that application and a slew of virtual drivers that are then used within Hubitat.
For sure nothing simple and quick.
I'll think about but I am not sure that I have enough free time right now to get started on this endeavor. Nor am I sure that all of this effort is worth it.....
