My question is about the hardware to be certified with Apple.
Of course, there may be other possibilities to use them without needing "Homebridge":
Last year there was an important announcement from Apple which fired some ideas in this community:
Anybody with good experience in using Particle devices with Homekit yet?
Currently, we don’t have plans to get certified by Apple. As Mesh is the primary use case for Particle Mesh, we intend to certify with the Thread Group, as well as other safetey and regulatory bodies (see full list here.)
As you mentioned, it should be possible to develop a library that implements the HomeKit Accessory Protocol - we’d love to see someone in the community take this on!
Quite a challenge... 
It’s actually quite easy to write plugins for homebridge. This is a one I found with limited accessory type: https://github.com/krvarma/homebridge-particle I made a private plugin that used particle’s js api and it worked well.
Yes, I imagine you can do it, but how many Particle users can say that?
Using Homebridge still requires running that server on the local network on a PC, Mac or Pi…
My question was, now that Apple opened up their program, how a Particle device can be used with Homekit, without Homebridge.
Lukas Jezny developed an application for a Photon, in which he implements a HAP (Homekit Access Point) on the device itself. So, you don’t need a Homebridge or an OpenHAB server to control a Photon with Siri… 
See this new thread: LINK