Not to defend their methodology, but your attempt to equate the GOP baseball shooting with an attempted coup says a lot more about your biases than the study authors'.
There was the CHAZ (Capitol Hill Autonomous Zone) established in Seattle where they explicitly seceded from the United States. In doing so, they pushed out all the police and set fire to the police station while barricading the doors with the remaining police inside (fortunately, the police escaped). The CHAZ rioters also occupied and vandalized City Hall.
Which has absolutely nothing to do with a full on coupe attempting to end our democracy by capturing the VP and Congress. There has always been civil unrest in the USA since the beginning. However never has a President attempted to overthrow and subvert the will of the people in a Presidential election through a violent mob. The two things only share one thing in common "two groups of people doing violence"
Seceding from the US is, literally, an insurrection.