Because media gaslighting works so well that people usually aren't aware that they've been affected by counterfactual narratives before they start actively reinforcing them by repetition.

The Google trends for this particular phrase:

Will the real terrorists please stand up?

Why not go with the much more common example of white supremacist

Because that'd be preaching to the choir. Trump voters don't care about white supremacist attacks.

what part of "thought exercise" do you not understand. its an extrapolation of current events, and hypothetical scenarios. not a statement of a factual event that has happened. You cant imagine this occuring?

Sources told NBC News Iran sent a communication to President Donald Trump in the days before Saturday night's strikes on its nuclear facilities, threatening to activate sleeper-cell terror inside the United States if it were attacked.

https://www.nbcchicago.com/news/local/department-of-homeland-security-terrorism-bulletin-threats-us-sleeper-cells-iran/3773879/