Lots of unathletic big men back then. Most had no skills other than being big. Fights were not uncommon because all the unathletic big men could do was take shots within 5 feet and muscle people. Most outside shooters were set shot shooters. A once or twice in a generation athletes could easily average 10 block shots a game during that era.
And eja, your argument about bigger men in a more athletic era with better athletes and different outside shooting doesn't hold water. Those stats that occurred 5-6 years after Russell retired were still a decade and more after Russell was in his prime. The game changes a lot in 10-15 years. Just look at how different the game is now as compared to the early 2000's. The three point shot wasn't close to being the weapon it is now and in the post big men are now dinosaurs. Game changes. So do stats.