Thanks for the helpful replies, all.
I guess those grades do sound kinda crazy. I live in a pretty mountainous area, with Mt Fuji and Mikuni Pass, the steepest segment of this year's Olympic men's road race, very close by.
One of my favorite roads up Mt Fuji, the Azami Line, is about 10% average grade, with some sections signposted between 12% and 20%:
https://pjammcycling.com/climb/520.M...Line-Bike-Ride
Mikuni Pass is also about 10% average grade, with some sections signposted 12 to 18%, although Japanese government sources say some part of the road exceeds 20% (
https://www.yamanashi-kankou.jp/zenr...ourse/007.html). It was entertaining to watch the Olympic men's race's leaders ride up that road in little more than half my best time. At least I'm beating Google Maps' estimate by a wide margin.
Mikuni Pass climb
I believe this is about where Tratnik began his slide from 1st position to ultimately finishing 67th.