The Homebridge project provides HomeKit compatibility without Apple authorization. I don't know if Apple controls this using patents or trademarks but you don't technically need Apples authorization.
I'm pretty sure that when you register a Homebridge instance to your Apple device though you get a message about the accessory being in test mode.
Homebridge is presumably making use of some debug mode that you wouldn't use in a shipping product - and if you did, Apple wouldn't let you advertise it as officially compatible with HomeKit.
From what I can tell from Homebridge they provide HomeKit emulation - not compatibility. You're running a local Node server to emulate the Homekit API.
Still neat - but hardly the sort of thing that implies you can ship a product utilizing Homekit without following the MFi spec.
To emulate the iOS HomeKit API, meaning you can write plugins for it to provide support for non-HomeKit-enabled devices. But presumably Homebridge itself must still vend itself as a HomeKit-enabled device on the network or else you wouldn't be able to control it.
