> How is the coup narrative so pervasive?
Because the fairly explicit goal was an autocoup.
> There was no institutional support
False.
> nobody got killed except for a rioter,
Even a successful coup is possible with no one getting killed;
> they all cleared out promptly by curfew
Because the coup attempt had already failed.
> By all indications it was a riot and a pretty mild one at that given the previous summer. How is that a coup?
“Coup” is not defined by intensity but by objective. The overtly intent was to use intimidation and/or violence against government officials to coerce a rejection of the electoral votes from sufficient states to allow the sitting President to extend his term nothwithstanding having been defeated in the election, at the instigation of the sitting President. It is a textbook autocoup attempt by its goals and ultimate instigation, which failed because the rioters were held back long enough for members of Congress to escape, but not by a wide margin.
Was it a hastily conceived, poorly coordinated, amateurish autocoup attempt? Yes, absolutely. Does that reduce its severity as a crime? No, no it doesn’t.
> I don't get how any honest assessment takes the actions by the partisans in DC as a coup.
It was a coup attempt, specifically an attempted autocoup. It wasn't a coup, because it failed.
> At LEAST the other riots were coups
No, none of those were coups, or even coup attempts. (The “Autonomous Zone” might be viewed as a hyperlocalized secession attempt, but that’s a distinct thing from a coup, seeking to separate territory from the control of an established government, not unlawfully take or extend control of said government.)