It lost its "cognitive immunity" long ago.
The time I really noticed the different realities was following 9/11, where a sort of madness happened. Setting aside the shock of the violence, many Americans were stunned that their self-image wasn't reflected in how foreign people saw the US.
To some extent, I think we are still sorting through Americans' bruised ego following that event.
But it isn't really cognitive immunity. It's intellectual cancer. And it started even earlier. I think Gingrich gave birth to it from a party viewpoint.
Maybe it was there before that, but I wasn't aware of it then, except in Talk Radio. But I just switched channels whenever Rush Limbaugh came on.