I don’t think you’ve read about the concept of “but for” in the law. Trump’s speech, and the lies he spread before and after the insurrection, are the reason the march on the capitol happened. Take him out of the equation and Giuliani, Powell, Flynn, etc. all have no reason to play their roles in the plot. Could it have happened without his approval and participation? Nobody else was president at the time, so his refusal to partake would have kiboshed the effort immediately, meaning it couldn’t have continued without him, meaning it wouldn’t have happened “but for” his involvement. He’s even admitted to his intent, at the time and after, being to hold on to power. It’s an airtight case.