I misread the last line of this when I first saw it... I thought 56% of all voters polled believed the election was stolen. Probably what it actually says is that 56% of people who believe the election was stolen think storming the capitol is OK.
These results suggest 30-40% of all voters think the election was stolen -- i.e., roughly 2x the number who approve of the storming: 0.21 / 0.56 ~ 38%
YouGov poll of 1,397 registered voters.