Please tell me that they are not replacing a knuckle! It's just a piece of cast iron with a machined seat for a hub bearing and machined ball joint perches.
The issue seems to be a wheel bearing, although I can't be sure. To change a bearing, they need to free the knuckle from steering or suspension restrictions (ball joints, etc) and pull the brake caliper, brake bracket, rotor, drive shaft spline and move the knuckle to a press where the hub, old bearing and clip are pushed out and a new bearing, hub and clip are pressed into the knuckle. The knuckle is then re-installed, drive and brake components re-installed an then aligned. 90-120 minutes for each wheel and an alignment.