I think that the manufacturer is concerned that if you bolt the engine to a very solid test mount, then all the vibration would be centered on the mounting ears and possibly break them. Most people I know don't run the engine on the ground very long at all and this shouldn't be an issue. I can't think of any need to run any gas engine on a test mount for 2 hours. My personal test mount is wood and will absorb some vibration similar to the way an airframe will absorb vibration when flying.