That is not true. The HomeKit protocol is documented. There are at least a few open source projects that implement it both as accessories and hubs. Home Bridge being the best known one.
If all the accessory vendors were to go under and Apple were to stop supporting HomeKit tomorrow, HomeKit accessories and hubs would still operate just fine and alternatives could be created. Even using Alexa or Google Nest for voice control.
Being documented is not enough. Implementing a commercial device requires including an Authentication component (before it used to be an actual chip, now it seems it can be done in software), and that can be used to prevent devices from speaking to alternative control apps.
From what I can tell, Homebridge implements Homekit as a device, receiving commands. But can anyone build an app or device to send commands, outside of iOS?
The fact that I can't find an Homekit control app for Android strongly indicates this is not possible.
> Being fully documented means anything can work in any way.
A protocol being fully documented does not mean you can get a device you cannot control (like an Homekit lightbulb) to connect to your server instead of the manufacturer's. If requests are signed and/or encrypted, knowing the protocol won't help if you don't have the keys. See also Tivoization.
> For example, search the web for ‘homebridge alexa’
I had found it. I don't think you realize that there's no Homekit there at all.
The point of Homebridge is to translate from a Homekit-speaking control app/assistant to a non-Homekit-enabled device:
But when you use homebridge-alexa, you're replacing that first link with another protocol (specifically, MQTT+JSON, which is what it uses to connect to https://www.homebridge.ca/).
So you've actually removed all Homekit parts from your smart home system!
If all the accessory vendors were to go under and Apple were to stop supporting HomeKit tomorrow, HomeKit accessories and hubs would still operate just fine and alternatives could be created. Even using Alexa or Google Nest for voice control.