Back in the summer Nest spokes people told 9to5mac they were "open to the idea" and are "considering homekit" due to the software based implementation that would allow them to take existing devices onto HomeKit without the need for hardware modifications.
So there is the possibility. However this could just be clever PR speak to prevent backlash of current and potential customers. Nest doesn't want their current customers (may of which use iPhones) to know anything about HomeKit and how they "could" support it if they chose too.
Their parent company (Google) maybe preventing them from doing this anyway. While it was drastically and massively help Nest gain a large segment of the smart device market across their product line it could hurt Google in the long run with any attempts of proprietary home automation service from them. Could just have been in the contract, "don't ever pay a direct competitor" and that "Works with Apple HomeKit" sticker on the box isn't free.
IMO for Nest its an asinine decision, so dumb that I can't even believe its theirs.
Lets face it, the people buying homekit stuff are the people with enough expendable income to afford a first world solution like a smart thermostat. And I'm not saying people that don't choose HomeKit don't have expendable income but the people that do already know (or quickly find out) their is a premium attached. Those are the people that companies like Nest need to market too!
I follow Nest and Ecobee on Twitter and common question for Nest is "is HomeKit coming?" "Does your thermostat support HomeKit?". And when Ecobee has a poll for why people chose them, HomeKit typically tops the chart by an overwhelming margin. There are just a lot of iPhone users.
I actually went to Home Depot with the intent to buy a Nest thermostat. For its price, quality and appearance I always assumed it was HomeKit compatible. After reading the box and quickly Google searching instead of spending $250 on a Nest I spent $170 on an Ecobee. And I still think the Nest is a better looking thermostat. Surely their marketing team knows that is a problem.
I've been avoid the Homebridge route (for now) because 1. I haven't NEEDED anything that required it. 2. I want manufacturers that utilize HomeKit to continue to see the value in it and further push product development.