The time machine argument is what it is. Modern players have modern advantages. That isn't just nutrition and equipment. McDavid has had professional skill and skating coaches since he was 6 years old.
Even with all that I'm not convinced it makes a big difference for superstars. I'm sure Gretzky had an advantage in terms of "software of training" by how obsessive he was about training at a young age, and with his father Walter, over McDavid or Crosby. Even Lemieux, playing against his older brother Alain (4 years older), which would explain a lot about his 1-on-1, cat and mouse abilities. A coach cannot replace those youth experiences.
Personally, I think the greatest variable is your morphology, down to the tiny angles of how your skeleton is distributed, which determines to a large extent how far at the extremes of a move you can go without committing, your dynamism and mobility, and how solid on your skates you are. Same with weight transfer for shot power. It's not about muscle, it's about structure, and structure is something you're born with. Sure, training will bonify it, but the foundation has to be there. Bobby Orr probably had a perfect skeletal shape to skate as great as he did.
In that sense, how many humans will have the perfect skeletal to play hockey will be random in any era. Could have 0 in the 1930's, 2 in the 1950's, 2 in the 1980's, 1 in the 2010's, 3 in the 2020's and 0 in the 2050's. The distribution of outliers won't be equal, and whatever advantages any era will give to the mass of players won't impact an outlier as much. What it does is make the average player better, but then the short shift game, with its emphasis on 100% effort, suffocates the data bank of creativity a player of old eras could accumulate. So the modern average player might be a better athlete, but have a weaker data bank about shift management and the likes.