Running a 2:21 marathon in Berlin (Shalane's PR) is absolutely amazing.
Running up and over a 3,000 foot pass, twice, between 10,000 and 13,000 feet at any speed is also amazing, especially when you're doing it between miles 38 and 62 at Leadville.
The thing is, the ONLY skill those two feats both require is substantial aerobic capacity. Otherwise, they are completely different sports. They require substantially different anatomic strengths and different physiological profiles.
Saying that Shalane would crush everyone at UTMB or Western States is like saying that LeBron James would be the world's best soccer goalie. Sure, it would be cool to see him try! If he could go back in time fifteen years and develop those skills instead of becoming the world's best basketball player, I'd be intrigued to see what he could have accomplished. But he didn't, just like Shalane hasn't spent 15 years trying to figure out how to ascend 18,000 feet on a 90-degree day at Western States while consuming 300 calories/hour.
Hammering out 5:20 miles over and over and over again on flat pavement is ridiculously amazing. It gives you a hell of a baseline to get into serious ultra training. But anyone who shows up at 100 mile mountain race with JUST that background (and there have been plenty of men with similar fitness profiles in ultrarunning) is going to have a bad time unless they are super conservative (which they probably won't be because the "right" pace is going to feel super slow to them.) Kara Goucher was three years out from a fourth place finish in the OT marathon when she ran ~9:00 pace for a trail marathon in Leadville. No, she wasn't anywhere close to her peak, but she wasn't a three hour marathoner, either. She was on mid-2:30s pace in Houson six months prior to Leadville before straining her hamstring.