2 records at once not possible
I posted some of the info below on another thread, but when looking at this it's very helpful to be aware of what the various performance calculators give as equivalent 100K marks for Walmsley's 4:50:08 (or so) 50-mile result, depending on the method:
• Riegel method equates 4:50:08 for 50 miles to 6:05:18 for 100K.
• Jack Daniels' VO2max method says it's worth 6:02:35.
Those are the two most optimistic assessments, and both are mathematically generated curves not derived (or at least not directly) from crunching WR statistics. The Purdy and Cameron methods, which are world-record-based, utilize WR performances to a certain depth (Cameron is based on top 10 all-time) to generate a statistical fit, if I understand things correctly.
• Purdy method gives an equivalent 100K of 6:11:35.
• Cameron method gives 6:21:25. (I erred in the other post saying Purdy/Cameron equivalents for 4:50:08 at 50 miles were *both* within about 2-3 minutes of the current 6:09:14 100K WR.)
Participation in ultra events has a shorter history and less depth of performances than in track and field, so WR-based methods of computing performance equivalence do not give the same confidence they do in shorter events like track and marathon, where many, many more athletes have been hammering away for longer with a larger athlete pool. My feeling is the Riegel and Daniel methods are therefore probably more realistic here.
For the sake of argument, say we start by using the range of 6:02 to 6:05 that those two methods give as equivalent 100K performances for Walmsley's 4:50:08. Based on that, it seems doubtful to me given the continually warming temperatures during the race that Jim could have broken 6 hours, even with better pacing. Because even then, he would have had an extra hour and 9 minutes to go to hit sub-6:00 with temperatures still rising. Carrie Tollefson mentioned on the live stream that temps were increasing about 4 degrees Fahrenheit every hour during the race. (A 24-degree increase from the start to a 6-hour finish.)
Considering what the conditions were, of course, Walmsley should be capable of running faster with cooler weather, which might well translate to a sub-6-hour 100k. But on the day, it doesn't look like that would have been possible.