Q: Apple TV - Siri - HomeKit

It appears the ATV is only acting as a server for your iOS device to control Homekit devices when away from home.  Using the Siri remote does not control the devices.

Use HomeKit-enabled accessories with your iPhone, iPad, and iPod touch - Apple Support

Only Apple TV development engineers would know, and I like you am just a user.

Your request does seem like a nice feature add for the next software update.  Make sure you submit it as an idea:

http://www.apple.com/feedback/appletv.html

I could even see a Apple TV commercial with the user saying Siri dim the lights, turn down the thermostat, and play 2001.  The response would be vocal and she would say Dave, sure I can do that.

I Use HomeKit quite a bit with 2 ATV 3s and one ATV 4 and can attest that siri the ATV4 has not been integrated with HomeKit. The device can function as a HomeKit hub but Siri has not had that software added to her knowledge base. We have seen Siri expand since release so we can only hope/expect that functionality will be added at some point. And as Vince suggests, sending feedback (as I have) will be the best we can do as the end users...

personally, I haven't been too impressed with Siri on ATV so far. I keep holding out hope though...