I don't recall commenting on the defensemen. But since you asked, the modern defenseman can skate better than most forwards. Of the defensemen you mentioned, let me just select Adam Foote. Slow.
Further, the quality of video and analytic preparation and the quality of coaching and defensive game planning is significant. Yea, old hockey was more brutal and a player like Kane might well have more trouble. Meanwhile there were star players in that era that couldn't keep up in this era. Tom Wilson would lap Cam Neely as an example.