I've toted my H.F. ac welder on the front rack of The Pig before. Sketchy at best but I got where I needed to...
It would have been much easier on a rear rack. I've carried adults on the back of a bike many, many times. Especially motorised bikes. It's totally practical to carry #2-300 pounds is you distribute it properly.
I travel self contained on my touring bike, but don't even carry close to what that cyclist is carrying. I might carry an extra 100lbs. Looks like he/she is carrying all of their worldly possessions; but then again, I've read where some first time coast-to-coast cyclist have started out with over 200lbs of gear and had to send some home after a few days.