Nicht Apple HomeKit wird Open Source, sondern Apple, Google und Amazon wollen Interoperabilität über einen neuen IP-basierten Standard erreichen.
Wenn ich das richtig verstehe, dann geht es eher um eine Meta-API (Open Source) um deren Dienste via IP miteinander reden zu lassen und Geräte einfacher zu integrieren.
Mehr hier:
https://www.connectedhomeip.com/
HomeKit Accessory Development Kit (ADK)
The HomeKit ADK is used by silicon vendors and accessory manufacturers to build HomeKit compatible devices.
The HomeKit ADK implements key components of the HomeKit Accessory Protocol (HAP), which embodies the core principles Apple brings to smart home technology: security, privacy, and reliability.
The HomeKit Open Source ADK is an open-source version of the HomeKit Accessory Development Kit. It can be used by any developer to prototype non-commercial smart home accessories. For commercial accessories, accessory developers must continue to use the commercial version of the HomeKit ADK available through the MFi Program.