"Enhancements" could mean improved/additional microphones or a dedicated chip for local processing. There's always a knee-jerk reaction of indignity when Apple restricts a new feature to new hardware, and it almost always turns out that there is a legitimate technical reason for the restriction.Honestly if they don't back-port this to the iPhone 7, 6S etc I'll be majorly pissed and might never buy another iPhone and I'm serious.
There is absolutely no reason on this earth an "enhanced siri" cannot be put on our older iPhones. We have the dual mics and processing power.