Once again, anyone that has ever run an ultra of 50 miles or longer (and to some extent 50k as well) fully understands that there is really probably one huge factor that makes them way different than other forms of distance running (in my opinion): nutrition. When you bonk in a marathon you just slow down for like 30 minutes to the finish. In an ultra when you bonk you could be in for 10 to 20 hours or longer of suffering if you don't fix it.
Pretty low chance Rupp gets nutrition or pacing right on his first hundred. Almost no one does and still runs to their potential. He could certainly get a big load on Walmsley at WS 100 but in Ultras even 2-3 hour leads are not safe. That time can be made up in one bad 10 mile section.
While distance running is still distance running, Ultras force you to have to deal with nutrition whereas no other distance running really does. For those that haven't run an ultra and say 8-9 minute pace sounds like an all day pace, it is until you throw in eating and fighting off nausea. 20 minute miles sound slow too until you are sick to your stomach going up some 45% grade mountain while it's 90F degrees.