If you just zapped forward anyone in just about any sport 90 years forward into an era where guys have modern fitness equipment, training regiments, steroids/HGH, and so on, I think they'd have problems. Really only boxing has this discussion I think where it is imagined by a significant number of people that the best of 100 or more years ago would flatten modern stars at any/all levels. You'd never hear anyone even consider suggesting this when it comes to the NBA or American Football.
I think you are correct in your assessment of other sports, but let's take a look at why we can make the comparisons in boxing. The NFL and NBA do not have weight limits as boxing does. A 210lb center in the 1940s now easily weighs 300lbs. Bob Cousy growing up in the 40s did not even know how to play basketball and "made up" the idea of a behind the back dribble and other advancements because it gave him an advantage. Now, even I can go behind the back with ease. But in boxing, a middleweight had to make 160 in 1920 as he does today. A lot of those advancements you mention are used as shortcuts today by fighters who generally do not stay in fighting shape throughout the year. Also, a hard body does not substitute for technique. When you consider that boxing had far more gyms and trainers and actual fighters in the past, I think you can see where the experience of regularly fighting and learning the craft can trump a part-time fighter of today even with their nutritional advancements.