That gel is mentioned one in the video, but your comment already provides more information than I know about it.

The video mainly talks about how (a) the second half-marathon was actually faster than the first half-marathon (b) any of the 5k segment times would be a world-class pace and (c) take any race length, calculate their average time for that length across the marathon, and it's damned impressive: their average 100m time was a good sprint time ad they did it nearly 430 times without stopping.

The author them matches their 400m pace for 400m, then collapses at 600m having already fallen off pace.