By this "logic" wasnt every player playing against AHL back ups?
It wasn’t just the goalies tho... most players of that era were crap. The talent gap between the top and bottom of the league was far larger than now.
Gretzky was certainly the best of his era, and still arguably the best ever respective of their era, but if you put Gretzky as he was size, training and all into today’s league, his fans would be disappointed. Hockey players are generally better today.
Edit: not to say he’d necessarily be bad.
I’d put it as... “hockey IQ” doesn’t change much over time. If you ‘get’ the game, if you can anticipate other players moves, you’ll be a better player. Skill does change.
If you apply the same IQ in a league with much less skill and hockey knowledge (due to less intense training), you can astronomically increase your points. Gretzky just got the game.