MacOS – Why Siri says that X is not on the find the friends list

For SMS, tell Siri "Text Alice on mobile I'll be right there."

Alternatively, you can set up two different contacts for Alice, one for SMS and one for iMessage and then tell Siri which one to use be citing the different names.

It's just a matter of hardware. The M9 coprocessor in the iPhone 6s is efficient enough to keep the microphone listening all the time without murdering the battery, unlike the chips in older devices.

From the article your question links to:

With iPhone 6s, the device is constantly monitoring audio input in the background, a process made possible by the highly efficient M9 motion coprocessor embedded in Apple's new A9 system-on-chip.