The reason I posted the video, complete with sarcastic comment, is precisely because you say stuff like that. "This small clip, IMO, doesn't look great." But you know for a fact that Jack Dempsey was and still is a legend of boxing. A truly dominant champion. So perhaps your view that the bag work is not impressive is flawed. And since the bag work here is not entirely dissimilar from Bruce's above, it should be evident that, in this regard, you have no room to speak about Bruce's evident technique.
Jack Dempsey appears to be working on his "whorl" here, his name for upper body rotation to add power to punches. It's a concept he discusses in his book, Championship Boxing. A book which Bruce is known to have used to form the basis of his Jeet Kune Do. So is it not possible that Bruce here is throwing power shots using the same technique? And is it also not possible that you, having probably never read Championship Boxing, or applied its techniques, don't know what Jack Dempsey's idea of good boxing looked like? And finally, is it not possible that you don't have grounds to challenge the efficacy of Jack Dempsey's punching style?
It may not look good to you. The point is, it doesn't matter if it looks good to you. That gives no credence whatsoever to the idea that it is ineffective. Clearly it was effective for Jack Dempsey, and that should at least prove that you can't write off Bruce's abilities on account of this video alone.
That being said, we'll never know how good Bruce actually was. It would have been great if he had competed. But it's hard to argue that he was not one of modern martial arts' greatest philosophers, and that's the truly important part of his legacy.