They don't say otherwise. Fewer goals scored doesn't mean better defense. It just means there are fewer goals being scored. It can mean weaker offense.
To put it differently, goals for and goals against are the same statistic, from different perspectives.
Offense and defense can both improve at the same time, which is what happened in the 1980s. But you can't have both more goals scored and fewer goals against, because they are the same thing.
Fair enough.
But I still don't buy it.
In the late 90's, goal totals dropped drastically. Was that not better defense, but rather better defense
and offense
?
Sounds like an argument for the game gets better because players are bigger, stronger, faster, more talented, coaches are better, more creative, equipment is better, etc.
Back to Bobby Hull, would he not have been as great playing in the 80's and 90's instead of the 60's and 70's?