Earlier quoted context omitted.
Yes they are trained (whether explicitly or informally between each other) to do this. It will continue so long as it works.
I'd like to ask you for a hypothetical. Say you are a police officer and one of your coworkers shouts "GUN" and you don't directly see one. You see your coworker freaking out, you see them drawing, and you see the other suspect moving. What do you do? Do you draw your gun/taser? I'd do that just going off of what my coworker has said. It's entirely reasonable to suspect there to be a gun. Now I'd like to run through…
There's a South Park sketch where the kids are out camping with the local gun nut, and he explains that they're not allowed to hunt most these animals - but they can act in self-defense. So he teaches them to shout "it's coming right for us!" before firing. ( https://www.youtube.com/watch?v=Nt6kKhlX8vU )
So what the grandparent is alluding to, isn't whether it's correct to act appropriately if "it's coming right for us!". It's the culture of "stop resisting" used as robotically as your Miranda rights to later justify an escalation of violence.
No-one's saying an officer shouldn't be allowed to respond to threats appropriately. We are saying we should be able to trust officers not to imagine threats for the sole purpose of justifying such an escalation (which, if taken at face value, is what the OP article describes).
And yes, you're right; other than taking the protagonist's word for it, it is exceptionally difficult to prove one from the other. That's what makes the prospect so troubling.