My local evening loop, 40km. Has been done in slightly shy of an hour (not these days!). On my own after work, and has around 450m of climbing, and includes a couple of km of footpath to start and finish. So not a huge stretch to see a group doing that sort of pace with no stops and no footpaths.
A decent group of fast 1st/2nd category type riders should be able to polish off that in 4 hours. If they work together.
What do the rest of the top 10-20 look like? If there is a stack of riders coming in at about the same time, minute gaps or something, having taken roughly the same time to complete, i’d guess that’s what it is. Tend to increase the pace at the end to get to the cafe first, some riders drop off and roll in on their own.
Edit, looks like you’ve answered that question.
Serves me right for wandering off to do work half way through typing a response.