They are convinced that's what happened due to the video and witness accounts of the event, as well as statements from those involved, and the pile of evidence pointing to there being some sort of at least loose idea or plan of what was supposed to take place, if not more than that.
Most video evidence that I’ve seen is of people either peacefully walking through the halls, or committing vandalism. I have yet to see any photo or video of anything “insurrection”-like occurring, besides that single photo of the guy with plastic handcuffs.
Most criminal charges are related to riot-like offenses (
2021 United States Capitol attack - Wikipedia , scroll down to charges), with conspiracy being the only one related to an “insurrection.” If there’s a “pile of evidence” showing that this was a carried-out, deliberate attempt to overthrow the government, why aren’t there charges reflecting that? It’s more likely that small amounts of this group discussed plans to attempt an insurrection, but didn’t carry it out, thus the charges of conspiracy but not of actual, physical, sedition.
That some want to frame it as a tourist event gone awry and are afraid of what an investigation might uncover as far as more evidence is the issue I'd say.
That’s projection. A fair and impartial investigation should be welcomed with everything, but bias before an investigation begins completely spoils the pot. Case in point, this whole mess with how biased scientists staged a coverup on the mere theory that Covid-19 was leaked from a lab, making even discussing it a “conspiracy theory.” The left supports fake news just as much as the right does, they just do it in different ways.
If you are telling me that a bunch of raucous antifa super soldiers stormed the capitol and injured a bunch of cops (maybe killing one since apparently that's up for debate), and caused lawmakers to suspend thier duties and flee to safe rooms, and security to draw weapons and open fire at one point during the act after a fiery speech by AOC insinuated she'd lead them there would not warrant an investigation and that's your stance.... Fine I guess, though I'd disagree, but I'm thinking for most people against an investigation, that's not at all the case and it's purely ass covering and damage control by the lawmakers on the offending side, and willful ignorance by supporters.
If your “Antifa super soldiers” had masks and guns and moved in groups with obvious intent to commit sedition, yeah, they should be investigated. Here, we have unarmed grandmas, unmasked, that were part of a riot. Until I see sedition charges and evidence of this dastardly coup attempt, its just left wing fantasy vs the evidence.