To clarify, this is about training your phone to recognize only "Hey Siri" as spoken by you, as opposed to letting you personalize your trigger phrase to something other than "Hey Siri".
The homepod knows to activate when I say hey siri, but my friends phone activates instead of the homepod whenever they say 'hey siri' about %50 of the time.
We experimented with this extensively.
We considered literally thousands of potential words. We also really didn't want anything like "hey" or "ok" to be required.
Note how long it took for "Computer" to be available - and you can imagine how many of us wanted that. I think it gives an idea of the difficulty.
I'd also be interested in the differences between a collectively-optimized wake word (without personal training) and one which the user personally trains against their voice à la the parent article. To what extent does that personal training supplement/supplant (or fail to do so) the "good enough for everyone" wake word kernels?
But thanks to the hubris of corporate branding and marketing departments we are unlikely to have this universally understandable command usable. Instead to use any light switch every consumer will need to ask what bloody set up the person has...