My Yanmar uses a paper gasket only. I will be swapping to a Speed Seal, which uses a rubber O ring and no paper gasket as you say. I didn't watch the entire video but from the knurled head of the cover screws it looks like it might be for a Speed Seal.

I paid a few extra bucks and got the 'deluxe' Speed Seal which comes with a recessed cover plate and a PTFE disc which spins along with the impeller. The company claims it extends impeller life, and reduces damage when running dry (see link).

I suspect the paper gasket or cover plate for my air leak. When I inspected it this summer there was a trace of a leak across the gasket. I replaced both the impeller and the paper gasket, but the problem continued. I decided to live with it until this fall since I'll be taking out the engine anyway and getting a chance to dive into it and put on the Speed Seal.