I think there's a difference between physically showing up at the Capitol during that event and being present for the whole thing (even if not entering the Capitol) versus just having an account on a site where some planning for the event took place. It seems to me that it would've been quite possible for a casual user of Parler -- or someone like the above poster who signed up as an observer -- to not even be aware of said planning. For instance, I'm pretty sure I have some family members who created Parler accounts in the past, and yet they called me up asking me what was going on at the Capitol and why, suggesting they were just as surprised/shocked as everyone else.
I think folks here are underestimating how many people just viewed Parler as a right-leaning Twitter and didn't realize how far some corners of it had went or were going.