Hmm. I don't know what to say. I feel silly saying that I disagree with a law professor about how sweeping this is, but I see nothing new.
What this citizen is doing is warning other citizens about the violation of law.
I don't even know what that means.
What the citizen flashing the light is doing is warning citizens that an LEO is nearby so that the citizen knows to STOP violating the law. It's a signal with a well-known meaning to the drivers in that area. It's no different--but probably less dangerous--than rolling down the window and yelling "LEO coming" or "Slow down."
Anyone ever been arrested for yelling slow down to a speeding driver? If they were arrested for that, would you expect them to get off on First Amendment grounds if they claimed that protection in court?
I don't get it.