Correct, the Apple TV is only to be able to use Siri while you're away from home. As far as I know, there are no HomeKit devices out there that don't require the Apple TV to give you Siri access away from home.I think you only need an Apple TV if you 1) Want to use Siri/HomeKit while you are away from home, and 2) You have pre-iOS 9 devices.
A.
That is my understanding as well that with IOS 9 it allows devices not to require AppleTV to communicate remotely - but it seems that these devices will require new hardware or maybe a software update to do this.Correct, the Apple TV is only to be able to use Siri while you're away from home. As far as I know, there are no HomeKit devices out there that don't require the Apple TV to give you Siri access away from home.
You'll always need the device's hub for it to work (Hue bridge, Caseta bridge). But you don't need an Apple TV if you only plan on using Siri at home. Just if you want to use it when you're away.
iOS 9 now allows devices to not need the Apple TV for remote Siri access, but I don't think any products have been released with that capability yet.