you don't need to run 100k race pace in training much to get efficient enough. Some marathoners never run at goal MP and it's no big deal. For guys like Tyler and Jim, you probably are doing some long runs around 5:30-6:00 anyway, so that's close enough. I know that Jim would do ~20 milers in training at Lake Mary or A1 road around 5:50/mi while doing ultra training, way before this Carbon X thing ever started.
As for figuring out a goal pace for 100k, it would be difficult. You sort of make an educated guess and then hope it works out. Tyler probably has no clue what he could sustain for 6 hours, but Jim has done plenty of trail 50 milers that take about 6 hours, so he knows exactly what effort he can sustain for 6 hours. if a road 100k also takes 6 hours, then he knows the exact effort. It's tougher when you are moving up in distance. I ran my first 50 miler this year and I looked a lot at previous results and strava data to figure out how fast runners of my calibre were running on the flat portions. There's certainly no accurate pace calculator or anything that you can use, unless the increase in distance is small - like going form 26.2 to 31.1 miles.
The pacers are supposed to go 20-30 miles, and you'd think that it wouldn't be much trouble to go the full 30.
Tyler is probably going to default to whatever pace Jim wants to run, which I heard was 3:36/km (5:48/mi, or 4:51 50mi or 6:00 100k pace). Tyler had wanted to aim for 3:30/km but that was targeting the 50mi record and not the 100k. Seems like Tyler cares more about the 50 miler and Jim the 100k.
a 6 hour run on roads is definitely different than 6 hours on trails, but it's a much simpler task in terms of strategy/fueling/hydration/gear. I'd love to be able to participate in an event like this. It's too bad they didn't open it up to anybody else, but I obviously understand why they wouldn't.