You mean the finest collection over-the-hill talent?
Derek Clayton and Ron Hill were two of the biggest names in that field, but they didn't run as well as expected. And neither runner ever performed at the top level again. Hill fared better than Clayton after Munich, as he ran 2:13 in 1973 and 2:12 and 2:13 in 1975.
The two other big names were Gaston Roelants and Mamo Wolde. Roelants was a good all rounder, but he was not a heavyweight in the marathon. He had won the World Cross Country Championship that year, but he wasn't on Shorter's level. Mamo Wolde, who was 40 years old, was the defending champion, but the '68 performance has to come with an asterisk since he won at altitude.
There were other well-known marathoners like Eckhard Lesse, Jack Foster, Dave McKenzie, Kenji Kimihara, and Terry Manners, but none was really on Shorter's level. Karel Lismont was not that well known at the time.
You're right that Shorter put distance on the field more because the competition died than because of any surge on his part. As I said, the field was relatively weak, although it was full of aging greats, and no one was able to keep up with Shorter after he took the lead at 15km, so he likely could have run faster if he had to.
Who are these historians you're referring to, anyway?