The steerer on this fork is 1-1/8" but seems to be steel or aluminum. Be sure you get a compression plug intended for a metal steerer, not a carbon one, as the ID's are different. "Locked in the preload with the stem" means once you adjust the headset with the top bolt, tightening the stem's steerer clamp bolts holds that setting. It's the same as your road bike.

If you haven't done this before, I strongly recommend you read Park Tool's web site tutorial on threadless headset installation and adjustment as it will describe all of the steps.