This is a good start in terms of adding some math to the discussion. I'm not totally sold on the adjusted stats from hockey-reference.com, but let's use them as a starting point. Here's a table showing the best five seasons for 25 of the top defensemen from the 1980's to today (plus Bobby Orr):
View attachment 744998
Karlsson ranks third, which is very impressive. Coffey is 14.7% ahead of him. Karlsson is only 4.3% ahead of 4th place (Leetch), 8.2% ahead of 5th place (Bourque), 11.9% ahead of 6th place (MacInnis), and 13.0% ahead of 7th place (Burns). (I just realized I didn't include Denis Potvin, who comes in at 79.0).
This is where the conversation turns to semantics, so I'll bow out. Does 15% qualify as "same tier"? I'll let the linguists debate that. But the size of the gap between Coffey and Karlsson (14.7%) is larger than the gap between Karlsson, Leetch, Bourque, Potvin, MacInnis and Burns. If we argue that Karlsson and Coffey are close, we'd have to conclude that Karlsson and Burns are close, and that doesn't feel right.
(EDIT - we should also consider the context and maybe an argument can be made that, considering team environments, Karlsson's lead over some of those players is more impressive than Coffey's lead over him. But it's still an uphill battle based on a lot of speculation).