As Mr. Midnight has said, the damper rod holder is introduced into the downtube cap end, and rotated slowly until you feel the socket drop-onto the damper rod top. Holding the rod stationary, remove the slider-end damper rod bolt thru the slider end hole.
Leaving the downtube threaded cap in-place, and then using an impact gun w/a proper allen head socket size into the slider end, is another way to approach the damper rod removal. In rare instances the damper rod may spin even though the downtube cap is in-place, and the spring is bearing onto the damper rod upper end. That's when you need to use the large socket to fit the damper rod upper end, closest to the handlebars. There's more than 1 way to skin that cat. If no-good for Plan A then Plan B.