Context matters. Karlsson had nowhere near the level of teammates Coffey had, nor did he play in the highest scoring era.
Playing in the 80s, with Gretzky, Messier, Kurri cannot be ignored. I don't think anyone disagrees with the fact that Coffey was a phenomenal player but if you're going to compare point totals, and difference in points between players etc. you have to do it in context. Karlsson never had the luxury of playing with the goat and 2 other hall of famers
First let me say that I think Karlsson was the best offensive defenseman in decades. But great teammates can cut both ways.
For most of his career Karlsson was the go to guy in Ottawa especially on the pp. If he was on the ice the puck went through him a lot. On the pp this was especially true. That means that if a goal is scored he probably gets a point simply because of the volume of touches even on plays where his contribution is not primary.
On the Oilers, as good as Coffey was, the guy who had the puck was Gretzky. Playing with Gretzky, Coffey was a target more than they guy stirring the pot once they were in the offensive zone. This was not because Coffey wasn't as incredibly talented offensively but more so because they had a guy was the best of all time. The Oilers power play back then never ran through its defencemen like most pp's do today. It was much more a forward focused attack.
An example of this is the impression that during their time together Nugent-Hopkins must have benefitted points-wise from playing with Hall who is perceived to be the better player. It turns out that at least 5 vs 5 this perception was wrong. Over the years they played together Nuge's ES scoring rate was actually higher away from Hall than it was with Hall. The reason was that Hall is a player who always wants the puck while when they played together Nuge deferred to Hall. Playing away from Hall Nuge was typically the go-to guy on his line and was far more often the puck carrier. Moreover, if you dig deeper it turns out that it was Nuge's assist rate that went down playing with Hall vs when he was playing with guys who typically scored much less, something that is completely counter intuitive. So while Coffey certainly benefitted substantially from playing with two of the games greats its not always as simple as the better the linemates the more you as an individual score. Karlsson with Gretzky would have had a very different role than he did in Ottawa. Certainly he would have put up more points, but I am less convinced that he would have been as good a complementary player for Gretzky as Coffey as was.
On a different note: . In Karlsson's 82 point season he had 56 ES points and no SH points. 36 of those were primary points. In Coffey's best year he had 30 ES and 9 SH goals. Plus 60 ES assists and 5 SH assists. 13 of his SH points were primary points and 69 of his ES points were primary. That's 81 primary points either ES or SH. I was at every home game Coffey played that year. He was beyond ridiculous. Other than Gretzky it was the best I ever saw anyone in an Oiler uniform play.