I know a downhill half marathon that gives you about 3-5 minutes of boost for the half. 80 minute guys run 72-73 minutes. The interesting thing is they claim the course is 2/3 uphill. So it makes me believe that your body gets used to the downhill and then you still struggle. Here's the course map with elevation.
https://www.halfmarathons.net/course-map-fontana-days-run-half-marathon/
Anyways, why did anyone think a no-name would run sub 2 for the marathon? Even on a downhill course that takes some serious talent to do. If you get 6 minutes of boost in total that still takes a 2:05 guy. 2:05 guys can make money doing a real marathon.