That footage is available to view,TBH I think Tunney was being kind,Corbett looked sprightly for his age ,but pretty basic.Tunney had taken the science of heavyweight pugilism to another level.He was also more durable and hit harder he beats Corbett just as he beat a fading Tom Gibbons ,clearly and conclusively.
Corbett was taller than Tunney and his speed would trouble Gene, but overall I believe that Tunney was simply better fighter.