Yeah that doesn't bother me. They aren't entering a concours.
I'm not a pro bike mechanic, but I find it hard to believe that engine is running a 15:1 or 16:1 compression like he said. I looked at a 1928 Mercedes-Benz SSK, a period performance street car ((the SSKL was the racing version) with a 431 cu. in. engine, seriously undersquare (100mm X 150mm) 2V inline-6 cyl, 247 hp @ 3300 rpm; 415 ft.lb. torque @ 1900 rpm; and supercharged, w/two carburetors. It runs a 6.2:1 compression ratio and burns gasoline. I would expect maybe 9:1 or 10:1 for the racing Harley-Davidson. The low octane gasoline of the time wouldn't allow high-compression engines like he claimed the H-D had.
I listened to the narrative again and heard him say that the bike runs alcohol, so that would allow a much-higher compression ratio than a gasoline-powered engine. The alcohol cools the engine as-well as it powers it.
The Offenhauser engine, an inline 4 cyl DOHC racing design, a derivative of the Miller racing engine of the same design, and which itself was supposed to be a copy of the pre-WW I (!) Peugeot DOHC Indy 500 racing design, was using alcohol to run high compression ratios. A road design car was built after WW II using the Offenhauser powerplant, designed by Frank Kurtis, the KK 500, the chassis could be had with or without fenders, and it was also a very-successful open-wheeled design in racing, where it used alcohol. When it was converted to using gasoline, it became problematic (the Offenhauser). The design was later sold as the Muntz Jet, and it didn't fare any-better in the marketplace. Changing names didn't change the problems of running gasoline in an engine designed to operate upon alcohol.
It's great to reunite the engine and chassis, and to have it operable. Maybe there are other videos of it underway.
I did research in my library, and I found in Allen Girdler's excellent reference (for those of you who don't know, Allen literally 'wrote the book' about the H-D racing motorcycle the XR-750) mentioning not the H-D DAR around 1928/'29, but a model called the DAH. He describes it as OHV (when H-D streetbikes wouldn't be OHV until the 1936 'Knuckleheads') and having 2V, but the distinctive pair of exhaust ports for each cylinder. The factory had already been using OHV engines for Class A competition, these were the pure-bred top-of-the-line racing machines from each manufacturer, and not required to be based on road-going production models. However, their road bikes sold to the public for transportation (or, in the case of Class C racing, competition) were IOE (intake over exhaust) or side-valve engines. The DAH was the correct displacement, 45 cu. in./750 cc V-twin. The pictures Girdler references are from H-D archives, taken in-front of the Juneau Ave. factory, and the story he relates is the same as Walksler's, that some of the bikes were sent to Europe for road-racing. The photograph Girdler's book mentions is not-included in the text, and he says it does show drum brakes, front and rear, and twin fenders. The Wheels Through Time "DAR" bike has no front fender and a bobbed rear fender. No biggie.
Some say the Class A bikes, especially the engines, were "loaned" to the competitors who were fortunate-enough to be entrusted to them. They were not sold, and the manufacturer could recall them at any time. Others say the Class A DAH was sold so there is some discrepancy, which Girdler's book mentions.
I suppose it's possible that after technology caught-up to and surpassed the level of development of the Class A machines, and with the passage of time, the bikes could have been 'surplused' by the factory (sold).