I can't find the exact place this happened in my strava but it was definitely within the first 30 minutes because the front pack of 50+ was still all together. I think it was more a clam downhill so everyone was relaxing for a sec and just not being able to see due to the density of riders. The video understates how close everyone was, it was one big pack like a road ride.
Oh you'd have good company, this ride always gets a huge turnout of regular riders since there are 3 length options for the rec ride with aid stations and a lot of support. If you want to try something similar the Sac O' Suds 50 is the first week of November and has a little more elevation but still pretty flat with good hardpack gravel.