This claim may be true, in that all the best 1000 mile times seem to have been posted during the 90s, just before Nedd was born. Many were splits from 1300 mile races.
Sri Chinmoy used to hold multiday festivals with 700 miles, 1000 miles and 1300 mile races. Naturally most of the best runners would run the longest race.
I know the UK 1000 mile record is held by Al Howie and it's a split from a 1300 mile race. I don't know why Sri Chinmoy hold events with unround distances like 1300 miles and 3100 miles. They are very close to 2000k and 5000k which would make more sense.
The most common multiday distance is 6 days, followed by 10 days. I can't think of a single world class multiday runner that has run a 1000 mile race since Nedd has been born, so while the claim may be true, it is a meaningless soundbite to attract publicity.
The only regular 1000 mile events over the last few years have been at the Italian Ultramarathon Festival in Policoro, Italy. But the fields were always very small and non elite, with the 6 and 10 day races having stronger fields.
A couple of race organisers in the US are talking about the possibility of organising a 1000 mile race, so we'll see what happens. If a good race organiser puts on a good 1000 mile race, I'm sure it will attract at least a couple of runners that would comfortably beat Brockmann.