To activate Siri on my iPhone 6, I have to actually double-tap the Home key. Otherwise, it won’t work.
Sword can keep me honest here, but I think you have to turn off Siri as a function in the general settings. I’ve had Siri actually turned off in my settings for years.
You can hold the phone up to your ear and say "Hey Siri. . . and proceed with your question. Or if you have a head set on, same thing. This "exploit" is so obscure, I doubt it will ever be a problem.
Actually it might - there’s a phone control in earbuds that work by changing the electrical resistance on the cable to get Siri to come up. This same process could probably fake it too.
Honestly, I don’t find this that big of a concern.