Oh, I believe it is real. I distinctly remember similar bikes participating in hill climbs. Back in the day, your dirt bikes did not have that much over street versions or scramblers in the area of suspension.
Many hills required speed and sheer power to make good time up a long steep climb that was fairly smooth. Small displacement engines had a tough time.
This guy looked like he just happened upon this event and thought he would show them how its done. Love it. Super video
Back in 71, had a neighbor who owned a shop making expansion chambers. His nephew, Raymond, came over from California. We were at hill climb outside of Whitney. While all these people were competing in Open class, Raymond took some "track papers" to the top, on a Yamaha DT250, just to the right of the track, in rougher virgin soil, and beat the open class rider to the top. Unknown to the crowd, Raymond was a California state hill climb champion. May have been 2nd place. Whatever, he was in tennis shoes, T-Shirt, and a stock Yamaha DT250 with dealer plates. Not even his bike. So, sometimes the rider can make anything sing!