The outer cap is what the 10mm bolt pushes against to self extract the crank from the splined shaft. I found it’s best if, before you undo the bolt, to try and squirt some grease or lubricant into the tiny gap around the inner ring of the cap so that the head of the bolt can slide against the inside of the cap. When undoing the, bolt it should release at first and for maybe half a turn then go tight when it touches the cap, which is when you may need an extension bar.
IME I’ve found that with a 12cm long Allen key there is enough torque to do it up by hand and stay tight without becoming prohibitively tight and ok to get undone without an extension bar.